Multi-level regulation of even-skipped stripes by the ubiquitous factor Zelda.

The zinc-finger protein Zelda (Zld) is a key activator of zygotic transcription in early Drosophila embryos. Here, we study Zld-dependent regulation of the seven-striped pattern of the pair-rule gene even-skipped (eve). Individual stripes are regulated by discrete enhancers that respond to broadly distributed activators; stripe boundaries are formed by localized repressors encoded by the gap genes. The strongest effects of Zld are on stripes 2, 3 and 7, which are regulated by two enhancers in a 3.8 kb genomic fragment that includes the eve basal promoter. We show that Zld facilitates binding of the activator Bicoid and the gap repressors to this fragment, consistent with its proposed role as a pioneer protein. To test whether the effects of Zld are direct, we mutated all canonical Zld sites in the 3.8 kb fragment, which reduced expression but failed to phenocopy the abolishment of stripes caused by removing Zld in trans. We show that Zld also indirectly regulates the eve stripes by establishing specific gap gene expression boundaries, which provides the embryonic spacing required for proper stripe activation.

Immune Reconstitution Inflammatory Syndrome After Hematopoietic Stem Cell Transplantation in a FOXN1 -deficient Patient.

The FOXN1 gene mutation is a unique disorder that causes the nude severe combined immunodeficiency phenotype. In patients with severe combined immunodeficiency, hematopoietic stem cell transplantation (HSCT) is life-saving if performed earlier. Thymic transplantation is the curative treatment for FOXN1 deficiency because the main pathology is thymic stromal changes. In this report, we describe the clinical features of a Turkish patient with a homozygous FOXN1 mutation treated with HSCT from his human leukocyte antigen-matched sibling. On follow-up, he showed Bacille Calmette Guerin adenitis and was evaluated as having immune reconstitution inflammatory syndrome. By presenting our patient, we aimed to draw attention to the development of HSCT and subsequent immune reconstitution inflammatory syndrome as a treatment option in patients with FOXN1 deficiency.

A snapshot of pediatric inpatients and outpatients with COVID-19: a point prevalence study from Turkey.

This multi-center point prevalence study evaluated children who were diagnosed as having coronavirus disease 2019 (COVID-19). On February 2nd, 2022, inpatients and outpatients infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) were included in the study from 12 cities and 24 centers in Turkey. Of 8605 patients on February 2nd, 2022, in participating centers, 706 (8.2%) had COVID-19. The median age of the 706 patients was 92.50 months, 53.4% were female, and 76.7% were inpatients. The three most common symptoms of the patients with COVID-19 were fever (56.6%), cough (41.3%), and fatigue (27.5%). The three most common underlying chronic diseases (UCDs) were asthma (3.4%), neurologic disorders (3.3%), and obesity (2.6%). The SARS-CoV-2-related pneumoniae rate was 10.7%. The COVID-19 vaccination rate was 12.5% in all patients. Among patients aged over 12 years with access to the vaccine given by the Republic of Turkey Ministry of Health, the vaccination rate was 38.7%. Patients with UCDs presented with dyspnea and pneumoniae more frequently than those without UCDs (p < 0.001 for both). The rates of fever, diarrhea, and pneumoniae were higher in patients without COVID-19 vaccinations (p = 0.001, p = 0.012, and p = 0.027).  Conclusion: To lessen the effects of the disease, all eligible children should receive the COVID-19 vaccine. The illness may specifically endanger children with UCDs. What is Known: • Children with COVID-19 mainly present with fever and cough, as in adults. • COVID-19 may specifically threaten children with underlying chronic diseases. What is New: • Children with obesity have a higher vaccination rate against COVID-19 than children without obesity. • Among unvaccinated children, fever and pneumoniae might be seen at a higher ratio than among vaccinated children.

In Situ Hybridization as a Method to Examine Gene Regulatory Activity In Vivo.

Transcription factor-enhancer binding events are among the most well-studied protein-DNA interactions, allowing researchers to determine mechanisms of transcriptional activation or repression during development. While large-scale ChIP-sequence datasets, together with computational predictions and chromatin accessibility data, yield information on potential transcription factor binding activities, reporter gene assays provide measurable information on whether these binding activities are functional in particular cell types during development. Here, we present a detailed protocol to examine enhancer activity in Drosophila embryos using cloning, transgenesis, and in situ hybridization.

The Notch1/CD22 signaling axis disrupts Treg function in SARS-CoV-2-associated multisystem inflammatory syndrome in children.

Multisystem inflammatory syndrome in children (MIS-C) evolves in some pediatric patients following acute infection with SARS-CoV-2 by hitherto unknown mechanisms. Whereas acute-COVID-19 severity and outcomes were previously correlated with Notch4 expression on Tregs, here, we show that Tregs in MIS-C were destabilized through a Notch1-dependent mechanism. Genetic analysis revealed that patients with MIS-C had enrichment of rare deleterious variants affecting inflammation and autoimmunity pathways, including dominant-negative mutations in the Notch1 regulators NUMB and NUMBL leading to Notch1 upregulation. Notch1 signaling in Tregs induced CD22, leading to their destabilization in a mTORC1-dependent manner and to the promotion of systemic inflammation. These results identify a Notch1/CD22 signaling axis that disrupts Treg function in MIS-C and point to distinct immune checkpoints controlled by individual Treg Notch receptors that shape the inflammatory outcome in SARS-CoV-2 infection.

Can the Prognosis of COVID-19 Disease Be Determined by Fecal Markers and Cytokines?

The coronavirus disease 2019 (COVID-19) pandemic has affected the entire world, and has a variety of clinical presentations. The aim of this study is to determine the relationships of fecal cytokines and markers with the symptoms and prognosis of children with COVID-19 infection, and to identify noninvasive markers during follow-up. In a cohort of 40 COVID-19-positive children and 40 healthy controls, fecal cytokines and markers were examined in stool samples. A binary logistic model was used to assess the potential of cytokines as risk factors for hospitalization. Odds ratios (ORs) with 95% confidence intervals (CIs) were reported. A P-value <0.05 was accepted as statistically significant. Levels of fecal lysozyme, myeloperoxidase, hemoglobin, and interleukin-5 (IL-5) (P < 0.05) were significantly higher among the patients than controls. In a logistic regression analysis, fecal IL-2 (OR = 3.83; 95% CI: 1.44-15.92), IL-4 (OR = 2.96; 95% CI: 1.09-12.93), IL-5 (OR = 4.56; 95% CI: 1.18-27.88), IL-10 (OR = 2.71 95% CI: 1.19-7.94), interferon-gamma (IFN-γ) (OR = 4.03; 95% CI: 1.44-15.73), IFN-α (OR = 3.02; 95% CI: 1.08-11.65), calcium-binding protein B S100 (S100 B) (OR = 4.78; 95% CI: 1.31-27.82), neutrophil elastase (NE) 2 (OR = 4.07; 95% CI: 1.17-19.69), and matrix metalloproteinase 1 (MMP-1) (OR = 3.67; 95% CI: 1.1-18.82) levels were significantly higher in hospitalized patients with COVID-19 infection than outpatients. We demonstrated that various fecal cytokines and markers were increased in patients who had COVID-19. Fecal IL-2, IL-4, IL-5, IL-10, IFN-γ, IFN-α, S100 B, NE, and MMP-1 levels were significantly elevated in hospitalized patients. We suggest that the fecal and serum levels of cytokines could be used to predict the prognosis of COVID-19 disease, although more studies are needed to confirm this.

COVID-19 Disease in Presenting to the Pediatric Emergency Department: A Multicenter Study of 8886 Cases.

BACKGROUND: The aim was to evaluate the epidemiological, clinical, laboratory, and radiologic data of children with SARS-CoV-2 positivity by polymerase chain reaction (PCR) together with treatment strategies and clinical outcomes and to evaluate cases of multisystem inflammatory syndrome in children (MIS-C) in this population. METHODS: This was a multicenter retrospective observational cohort study performed in the pediatric emergency departments of 19 tertiary hospitals. From March 11, 2020, to May 31, 2021, children who were diagnosed with confirmed nasopharyngeal/tracheal specimen SARS-CoV-2 PCR positivity or positivity for serum-specific antibodies against SARS-CoV-2 were included. Demographics, presence of chronic illness, symptoms, history of contact with SARS-CoV-2 PCR-positive individuals, laboratory and radiologic investigations, clinical severity, hospital admissions, and prognosis were recorded. RESULTS: A total of 8886 cases were included. While 8799 (99.0%) cases resulted in a diagnosis of SARS-CoV-2 with PCR positivity, 87 (1.0%) patients were diagnosed with MIS-C. Among SARS-CoV-2 PCR-positive patients, 51.0% were male and 8.5% had chronic illnesses. The median age was 11.6 years (IQR: 5.0-15.4) and 737 (8.4%) patients were aged <1 year. Of the patients, 15.5% were asymptomatic. The most common symptoms were fever (48.5%) and cough (30.7%) for all age groups. There was a decrease in the rate of fever as age increased (p < 0.001); the most common age group for this symptom was <1 year with the rate of 69.6%. There was known contact with a SARS-CoV-2 PCR-positive individual in 67.3% of the cases, with household contacts in 71.3% of those cases. In terms of clinical severity, 83 (0.9%) patients were in the severe-critical group. There was hospital admission in 1269 (14.4%) cases, with 106 (1.2%) of those patients being admitted to the pediatric intensive care unit (PICU). Among patients with MIS-C, 60.9% were male and the median age was 6.4 years (IQR: 3.9-10.4). Twelve (13.7%) patients presented with shock. There was hospital admission in 89.7% of these cases, with 29.9% of the patients with MIS-C being admitted to the PICU. CONCLUSION: Most SARS-CoV-2 PCR-positive patients presented with a mild clinical course. Although rare, MIS-C emerges as a serious consequence with frequent PICU admission. Further understanding of the characteristics of COVID-19 disease could provide insights and guide the development of therapeutic strategies for target groups.

Notch1-CD22-Dependent Immune Dysregulation in the SARS-CoV2-Associated Multisystem Inflammatory Syndrome in Children.

Multisystem inflammatory syndrome in children (MIS-C) evolves in some pediatric patients following acute infection with SARS-CoV-2 by hitherto unknown mechanisms. Whereas acute-COVID-19 severity and outcome were previously correlated with Notch4 expression on regulatory T (Treg) cells, here we show that the Treg cells in MIS-C are destabilized in association with increased Notch1 expression. Genetic analysis revealed that MIS-C patients were enriched in rare deleterious variant impacting inflammation and autoimmunity pathways, including dominant negative mutations in the Notch1 regulators NUMB and NUMBL. Notch1 signaling in Treg cells induced CD22, leading to their destabilization in an mTORC1 dependent manner and to the promotion of systemic inflammation. These results establish a Notch1-CD22 signaling axis that disrupts Treg cell function in MIS-C and point to distinct immune checkpoints controlled by individual Treg cell Notch receptors that shape the inflammatory outcome in SARS-CoV-2 infection.

Comparisons of Clinical Features and Outcomes of COVID-19 between Patients with Pediatric Onset Inflammatory Rheumatic Diseases and Healthy Children.

(1) Background: We aimed to describe the clinical features and outcomes of coronavirus disease-2019 (COVID-19) in children and late adolescents with inflammatory rheumatic diseases (IRD) and to measure their severity risks by comparing them with healthy children. (2) Methods: Among children and late adolescents found to be severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) positive via polymerase chain reaction (PCR) test, IRD patients with an at least six-months follow-up duration, and healthy children were included in the study. Data were obtained retrospectively. (3) Results: A total of 658 (339 (51.5%) females) (healthy children: 506, IRD patients: 152) subjects were included in the study. While 570 of 658 (86.6%) experienced COVID-19-related symptoms, only 21 (3.19%) required hospitalization with a median duration of 5 (1-30) days. Fever, dry cough, and fatigue were the most common symptoms. None of evaluated subjects died, and all recovered without any significant sequelae. The presence of any IRD was found to increase the risk of both hospitalization (OR: 5.205; 95% CI: 2.003-13.524) and symptomatic infection (OR: 2.579; 95% CI: 1.068-6.228). Furthermore, increasing age was significantly associated with symptomatic infection (OR: 1.051; 95% CI: 1.009-1.095). (4) Conclusions: Our study emphasizes that pediatric rheumatologists should monitor their patients closely for relatively poor COVID-19 outcomes.

Diagnostic and Prognostic Biomarkers of Coronavirus Disease 2019 in Children.

Predictors of early diagnosis and severe infection in children with coronavirus disease 2019 (COVID-19), which has killed more than 4 million people worldwide, have not been identified. However, some biomarkers, including cytokines and chemokines, are associated with the diagnosis, pathogenesis and severity of COVID-19 in adults. We examined whether such biomarkers can be used to predict the diagnosis and prognosis of COVID-19 in pediatric patients. Eighty-nine children were included in the study, comprising three patient groups of 69 patients (6 severe, 36 moderate and 27 mild) diagnosed with COVID-19 by real-time polymerase chain reaction observed for 2-216 months and clinical findings and 20 healthy children in the same age group. Hemogram, coagulation, inflammatory parameters and serum levels of 16 cytokines and chemokines were measured in blood samples and were analyzed and compared with clinical data. Interleukin 1-beta (IL-1ß), interleukin-12 (IL-12) and interferon gamma-induced protein 10 (IP-10) levels were significantly higher in the COVID-19 patients (p = 0.035, p = 0.006 and p < 0.001). Additionally, D-dimer and IP-10 levels were higher in the severe group (p = 0.043 for D-dimer, area under the curve = 0.743, p = 0.027 for IP-10). Lymphocytes, C-reactive protein and procalcitonin levels were not diagnostic or prognostic factors in pediatric patients (p = 0.304, p = 0.144 and p = 0.67). Increased IL-1ß, IL-12 and IP-10 levels in children with COVID-19 are indicators for early diagnosis, and D-dimer and IP-10 levels are predictive of disease severity. In children with COVID-19, these biomarkers can provide information on prognosis and enable early treatment.

Coronavirus infections in childhood and vaccine studies.

In late December 2019, a new coronavirus (CoV) called the severe acute respiratory syndrome CoV 2 (SARS-CoV-2), which had not been detected in humans before, caused a worldwide pandemic. Owing to the highly infectious nature of this virus, it spread rapidly from person to person despite the warnings of the World Health Organization and all the measures taken by the governments. Although it has been reported that SARS-CoV-2 is more likely to infect the elderly, all age groups are susceptible to this virus, including newborns. CoV disease 2019 (COVID-19) symptoms seem to be less severe in children than in adults, but similar to the 2003 severe acute respiratory syndrome epidemic, in the COVID-19 pandemic, the number of cases and the risk of serious diseases increase as age increases. The treatment of COVID-19 is still challenging, especially in children, and the virus continues to cause death worldwide. The safest and most controlled way to effectively and sustainably prevent COVID-19 in a society is to have an effective and safe vaccine and to successfully vaccinate the majority of the population. It is possible that vaccines with safety and efficacy that have been proven in phase III trials will be effective in handling COVID-19.

Multicenter prospective surveillance study of viral agents causing meningoencephalitis.

The frequency of bacterial factors causing central nervous system infections has decreased as a result of the development of our national immunization program. In this study, it is aimed to obtain the data of our local surveillance by defining the viral etiology in cases diagnosed with meningoencephalitis for 1 year. Previously healhty 186 children, who applied with findings suggesting viral meningoencephalitis to 8 different tertiary health centers between August 2018 and August 2019, in Istanbul, were included. The cerebrospinal fluid (CSF) sample was evaluated by polymerase chain reaction. The M:F ratio was 1.24 in the patient group, whose age ranged from 1 to 216 months (mean 40.2 ± 48.7). Viral factor was detected in 26.8%. Enterovirus was the most common agent (24%) and followed by Adenovirus (22%) and HHV type 6 (22%). In the rest of the samples revealed HHV type 7 (10%), EBV (6%), CMV (6%), HSV type 1 (6%), Parvovirus (4%) and VZV (2%). The most common symptoms were fever (79%) and convulsions (45.7%). Antibiotherapy and antiviral therapy was started 48.6% and 4% respectively. Mortality and sequela rate resulted 0.53% and 3.7%, respectively. This highlights the importance of monitoring trends in encephalitis in Turkey with aview to improving pathogen diagnosis for encephalitis and rapidly identifying novel emerging encephalitis-causing pathogens that demand public health action especially in national immunisation programme.

Suboptimal Intermediates Underlie Evolution of the Bicoid Homeodomain.

Changes in regulatory networks generate materials for evolution to create phenotypic diversity. For transcription networks, multiple studies have shown that alterations in binding sites of cis-regulatory elements correlate well with the gain or loss of specific features of the body plan. Less is known about alterations in the amino acid sequences of the transcription factors (TFs) that bind these elements. Here we study the evolution of Bicoid (Bcd), a homeodomain (HD) protein that is critical for anterior embryo patterning in Drosophila. The ancestor of Bcd (AncBcd) emerged after a duplication of a Zerknullt (Zen)-like ancestral protein (AncZB) in a suborder of flies. AncBcd diverged from AncZB, gaining novel transcriptional and translational activities. We focus on the evolution of the HD of AncBcd, which binds to DNA and RNA, and is comprised of four subdomains: an N-terminal arm (NT) and three helices; H1, H2, and Recognition Helix (RH). Using chimeras of subdomains and gene rescue assays in Drosophila, we show that robust patterning activity of the Bcd HD (high frequency rescue to adulthood) is achieved only when amino acid substitutions in three separate subdomains (NT, H1, and RH) are combined. Other combinations of subdomains also yield full rescue, but with lower penetrance, suggesting alternative suboptimal activities. Our results suggest a multistep pathway for the evolution of the Bcd HD that involved intermediate HD sequences with suboptimal activities, which constrained and enabled further evolutionary changes. They also demonstrate critical epistatic forces that contribute to the robust function of a DNA-binding domain.

COVID-19 in Turkey: A tertiary center experience.

BACKGROUND: Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has caused a serious epidemic in our country and all over the world since December 2019 and has become a global health problem. The disease caused by the SARS-CoV-2 virus has been named as coronavirus disease 19 (COVID-19). METHODS: We report on the epidemiological and clinical features of 37 children diagnosed with COVID-19. RESULTS: The median age was of the children was 10 years and 57.1% were male. In addition, 78.3% of the children had a history of contact with adult patients who had been diagnosed with COVID-19, and 27.0% had coexisting medical conditions. We found that 40.5% of our patients had mild infection, while 32.4% had moderate infection, and 27.1% had developed severe or critical illness. The most common abnormal laboratory findings in our patients were decreased lymphocytes (45.9%) and increased D-dimer values (43.2%), while abnormal radiological findings were detected in 56.7% of the children. In addition, 64.8% of the children had received azithromycin, 59.4% had received oseltamivir, and hydroxychloroquine was used in combination with azithromycin in 35.1% of the children. Non-invasive mechanical ventilation was required in 27.0% of the children. CONCLUSIONS: Although COVID-19 infection is usually mild in children, severe illness can be seen in children with comorbidities, or even in children who were previously healthy.

Determination of tuberculin skin test for isoniazid prophylaxis in BCG vaccinated children who are using anti-TNF agents for rheumatologic diseases.

OBJECTIVE: The use of tumor necrosis factor inhibitors (anti-TNF) has a risk of activating latent tuberculosis infection (LTBI). This study was performed to investigate LTBI according to tuberculin skin test (TST) size and to determine the frequency of tuberculosis (TB) in bacillus Calmette-Guerin (BCG)-vaccinated children receiving anti-TNF treatment for rheumatological disease. MATERIALS AND METHODS: The study consisted of 559 children. Information on demographics, anti-TNF agents, TST size, and isoniazid (INH) prophylaxis was recorded. Patients (n = 254) with TST size ≥5 mm were divided into three groups according to TST size and INH prophylaxis: group 1, TST size 5 to 9 mm and no INH prophylaxis; group 2, TST size 5 to 9 mm with INH prophylaxis; and group 3, TST size ≥10 mm with INH prophylaxis. RESULTS: The 559 patients comprised 314 (56.3%) females and 245 (43.6%) males; they had a mean age of 13.1 ± 4.1 years. The mean TST size in all patients was 4.2 ± 4.7 mm. Group 1 consisted of 76 (29.9%) patients, group 2 consisted of 88 (34.6%) patients, and group 3 consisted of 90 (35.4%) patients. The mean TST sizes for the three groups were 6.8 ± 3.1 mm, 7.2 ± 3.2 mm, and 13.9 ± 2.8 mm, respectively. New TB was diagnosed in only two (0.35%) patients. Both of them were in group 3. CONCLUSIONS: A TST size of ≥10 mm in BCG-vaccinated children receiving anti-TNF treatment may distinguish children at high risk for reactivation of LTBI.

SARS-CoV-2 infection in children.

SARS-CoV-2, a RNA virus that emerged in December 2019 in the city of Wuhan in China and took hold of the whole world, affects children as well as all age groups. In our country, we started to observe the first cases by March 2020. SARS-CoV-2, which is transmitted by droplets and by way of contact with surfaces contaminated by these droplets, is generally transmitted to children from adults through close contact. There is no proven information about other transmission routes such as fecal-oral transmission. Similar to adults, the primary symptoms at presentation include fever, cough, sore throat, malaise, nasal discharge, and rarely, vomiting and diarrhea in children. Although the majority of pediatric patients are asymptomatic or have a mild clinical course, severe cases have been reported in children with underlying chronic diseases. There is currently no specific antiviral treatment against the SARS-CoV-2 virus. Supportive treatment is recommended in children with a mild course, and some treatments are recommended in children with comorbidities or in children who are observed to have a more severe course. Asymptomatic pediatric patients or pediatric patients who have a mild course constitute an important group in terms of transmission of the infection to the advanced age group who carry high risk. Prevention of infection is very important in terms of reducing new cases and alleviating the load on the healthcare system. In order to prevent transmission of SARS-CoV-2, hygienic rules should be pursued in the community, social distancing should be observed, and the family members and contacts of patients who have been diagnosed should be screened and isolated.

Ancient mechanisms for the evolution of the bicoid homeodomain's function in fly development.

The ancient mechanisms that caused developmental gene regulatory networks to diversify among distantly related taxa are not well understood. Here we use ancestral protein reconstruction, biochemical experiments, and developmental assays of transgenic animals carrying reconstructed ancestral genes to investigate how the transcription factor Bicoid (Bcd) evolved its central role in anterior-posterior patterning in flies. We show that most of Bcd's derived functions are attributable to evolutionary changes within its homeodomain (HD) during a phylogenetic interval >140 million years ago. A single substitution from this period (Q50K) accounts almost entirely for the evolution of Bcd's derived DNA specificity in vitro. In transgenic embryos expressing the reconstructed ancestral HD, however, Q50K confers activation of only a few of Bcd's transcriptional targets and yields a very partial rescue of anterior development. Adding a second historical substitution (M54R) confers regulation of additional Bcd targets and further rescues anterior development. These results indicate that two epistatically interacting mutations played a major role in the evolution of Bcd's controlling regulatory role in early development. They also show how ancestral sequence reconstruction can be combined with in vivo characterization of transgenic animals to illuminate the historical mechanisms of developmental evolution.

Gene expression of pluripotency determinants is conserved between mammalian and planarian stem cells.

Freshwater planaria possess extreme regeneration capabilities mediated by abundant, pluripotent stem cells (neoblasts) in adult animals. Although planaria emerged as an attractive in vivo model system for stem cell biology, gene expression in neoblasts has not been profiled comprehensively and it is unknown how molecular mechanisms for pluripotency in neoblasts relate to those in mammalian embryonic stem cells (ESCs). We purified neoblasts and quantified mRNA and protein expression by sequencing and shotgun proteomics. We identified ∼4000 genes specifically expressed in neoblasts, including all ∼30 known neoblast markers. Genes important for pluripotency in ESCs, including regulators as well as targets of OCT4, were well conserved and upregulated in neoblasts. We found conserved expression of epigenetic regulators and demonstrated their requirement for planarian regeneration by knockdown experiments. Post-transcriptional regulatory genes characteristic for germ cells were also enriched in neoblasts, suggesting the existence of a common ancestral state of germ cells and ESCs. We conclude that molecular determinants of pluripotency are conserved throughout evolution and that planaria are an informative model system for human stem cell biology.

Adenosine kinase of Trypanosoma brucei and its role in susceptibility to adenosine antimetabolites.

Trypanosoma brucei cannot synthesize purines de novo and relies on purine salvage from its hosts to build nucleic acids. With adenosine being a preferred purine source of bloodstream-form trypanosomes, adenosine kinase (AK; EC 2.7.1.20) is likely to be a key player in purine salvage. Adenosine kinase is also of high pharmacological interest, since for many adenosine antimetabolites, phosphorylation is a prerequisite for activity. Here, we cloned and functionally characterized adenosine kinase from T. brucei (TbAK). TbAK is a tandem gene, expressed in both procyclic- and bloodstream-form trypanosomes, whose product localized to the cytosol of the parasites. The RNA interference-mediated silencing of TbAK suggested that the gene is nonessential under standard growth conditions. Inhibition or downregulation of TbAK rendered the trypanosomes resistant to cordycepin (3'-deoxyadenosine), demonstrating a role for TbAK in the activation of adenosine antimetabolites. The expression of TbAK in Saccharomyces cerevisiae complemented a null mutation in the adenosine kinase gene ado1. The concomitant expression of TbAK with the T. brucei adenosine transporter gene TbAT1 allowed S. cerevisiae ado1 ade2 double mutants to grow on adenosine as the sole purine source and, at the same time, sensitized them to adenosine antimetabolites. The coexpression of TbAK and TbAT1 in S. cerevisiae ado1 ade2 double mutants proved to be a convenient tool for testing nucleoside analogues for uptake and activation by T. brucei adenosine salvage enzymes.