Mutational hotspot in the human biotinidase gene causes profound biotinidase deficiency.

Biotinidase deficiency is an autosomal recessive inherited disorder that is characterized by neurological and cutaneous symptoms. Biotinidase-deficient children cannot recycle endogenous biotin, an essential water-soluble B vitamin. Biotin is covalently attached to epsilon-amino groups of lysyl residues of four carboxylases. These carboxylases are subsequently degraded to biocytin (biotin-epsilon-lysine). Biotinidase cleaves biocytin to biotin and lysine, thereby completing the biotin cycle. The symptoms of biotinidase deficiency can be resolved or prevented by treatment with biotin. Therefore, it is important that biotinidase deficiency is diagnosed early so that permanent neurological damage can be prevented. Many states and countries currently perform newborn screening for biotinidase deficiency. We have recently isolated and characterized the cDNA for normal human biotinidase and localized the gene to chromosome 3p25 (ref. 9). We have now identified the first mutation that causes profound biotinidase deficiency. It occurs in a distinct region of the gene that encodes the putative signal peptide. Fifty percent of symptomatic children studied have a 7-bp deletion coupled with a 3-bp insertion in at least one of their alleles of the biotinidase gene. This mutation appears to be a common cause of biotinidase deficiency in symptomatic children.

The scavenger receptor MARCO is involved in Leishmania major infection by CBA/J macrophages.

CBA/J mice are resistant to Leishmania major infection but are permissive to L. amazonensis infection. In addition, CBA/J macrophages control L. major but not L. amazonensis infection in vitro. Phagocytosis by macrophages is known to determine the outcome of Leishmania infection. Pattern recognition receptors (PRR) adorning antigen presenting cell surfaces are known to coordinate the link between innate and adaptive immunity. The macrophage receptor with collagenous structure (MARCO) is a PRR that is preferably expressed by macrophages and is capable of binding Gram-positive and Gram-negative bacteria. No research on the role of MARCO in Leishmania-macrophage interactions has been reported. Here, we demonstrate, for the first time, that MARCO expression by CBA/J macrophages is increased in response to both in vitro and in vivo L. major infections, but not to L. amazonensis infection. In addition, a specific anti-MARCO monoclonal antibody reduced L. major infection of macrophages by 30%-40% in vitro. The draining lymph nodes of anti-MARCO-treated mice displayed a reduced presence of immunolabelled parasite and parasite antigens, as well as a reduced inflammatory response. These results support the hypothesis that MARCO has a role in macrophage infection by L. major in vitro as well as in vivo.

Characterization of the rRNA-encoding genes and transcripts, and a group-I self-splicing intron in Pneumocystis carinii.

Although Pneumocystis carinii is the most common opportunistic pathogen infecting individuals with AIDS, very little is known of the basic biology of the organism. We have examined the ribosomal RNA (rRNA) and the DNA encoding it (rDNA) in P. carinii in an attempt to clarify its taxonomic position and to begin to study its genetic processes. Electrophoretic analysis showed that the sizes of the P. carinii rRNAs are quite similar to the sizes of the corresponding rRNAs from Saccharomyces cerevisiae. Direct sequence analysis of approx. 60% of the 18S small subunit-rRNA (Ss-rRNA) confirmed that its sequence is similar to that of yeast-like fungi and that a putative group-I intron previously observed in the 18S rDNA is, in fact, excised from the mature rRNA. PCR analysis of the intron in P. carinii genomic DNA showed that each of the multiple rDNA genes bears the group-I intron and in vitro transcripts of the intron autocatalytically excise from the rRNA primary transcript in the presence of GTP. Finally, analogues of GTP inhibit the self-splicing reaction, indicating that the guanosine-binding site of the intron closely resembles that of other well-characterized group-I introns. Since no group-I introns have been found in higher eukaryotes, this self-splicing process represents a viable target for chemotherapy of P. carinii pneumonia (PCP).

Re-expression of an inactivated variable surface glycoprotein gene in Trypanosoma equiperdum.

Variable surface glycoprotein (VSG) genes in African trypanosomes are often activated by the duplicative transposition of a silent basic copy (BC) gene into an unlinked telomerically located expression site, producing an active expression-linked copy (ELC) of that gene. However, some BC genes that are already linked to a telomere are activated without apparent duplication or transposition. We have recently shown that an active VSG ELC can be inactivated in situ, apparently without rearrangement. To explain these observations it has been suggested that VSG genes that are associated with chromosome telomeres are activated by chromosome end exchanges that occur at a considerable distance upstream from the genes themselves and place them cis to a unique VSG expression element. In an attempt to test this model we derived five VSG-1 expressing variants from BoTat-2, a VSG-2 expressing variant of Trypanosoma equiperdum which carries an inactive residual VSG-1 ELC (R-ELC) as well as the active VSG-2 ELC near unlinked chromosome telomeres. We examined the fates of the VSG-2 ELC and the VSG-1 R-ELC in these variants. All five had maintained the VSG-1 R-ELC; three in a reactivated form and two in an inactive state. The latter two variants carried new, active VSG-1 ELCs: one in the site that had previously contained the VSG-2 ELC and one in a previously unidentified site. The VSG-2 ELC was lost in all five of the variants. The results are not consistent with the simple chromosome end exchange model, which predicts that the VSG-2 ELC would be inactivated but not deleted when the VSG-1 R-ELC was reactivated.

Characterization of the spliced leader genes and transcripts in Trypanosoma cruzi.

Trypanosome mRNA is processed to maturity in a novel trans-splicing reaction during which a 35-nucleotide (nt) spliced leader (SL) is joined to the 5' ends of most structural gene transcripts. We have examined this process in Trypanosoma cruzi, the causative agent of Chagas' disease in Central and South America. In this communication, we characterize the genes encoding the SL (SL gene) in five different strains of T. cruzi by hybridization analysis and show that the genome of each of these strains contains numerous tandemly repeated copies of the SL gene. We demonstrate that the SL genes show remarkable intrastrain homogeneity, but significant interstrain heterogeneity. We have cloned and sequenced one of the SL repeats from T. cruzi strain CL and used synthetic oligodeoxyribonucleotides designed to hybridize to SL gene transcripts in Northern analyses of T. cruzi RNA to identify an approx. 110-nt putative SL primary transcript (SL-RNA). The 5' end of the SL-RNA was mapped to the first nt of the SL by primer extension analyses. The sequence of the 110-nt SL-RNA was used to generate a predicted secondary structure, and this structure compared favorably to the predicted secondary structures of SL transcripts of other trypanosomatids.

The transcription promoter of the spliced leader gene from Trypanosoma cruzi.

A putative promoter element responsible for transcription of the spliced leader (SL) gene of Trypanosoma cruzi was identified by overlapping deletion and linker scanning analyses of the upstream flanking sequences using the bacterial chloramphenicol acetyltransferase (CAT) gene as a reporter in transient transfections of cultured epimastigotes. Deletion or substitution of a proximal sequence element (PSE) between positions -53 and -40 relative to the transcription start point eliminated CAT gene expression. Comparison of SL genes from several strains of T. cruzi revealed two alternative sequence patterns for the putative SL PSE, both composed of a short run of purines followed by a run of pyrimidines. Moreover, an examination of these sequences supports the subdivision of T. cruzi isolates into two divergent groups. Double-stranded oligonucleotides containing the sequence of the PSE exhibited specific gel mobility shifts after incubation with T. cruzi nuclear extracts, suggesting that a transcription factor binds this site. Finally, experiments designed to increase the level of CAT expression from the SL promoter suggest that it is not a strong promoter in cultured T. cruzi epimastigotes.

Expression of an exogenous gene in Trypanosoma cruzi epimastigotes.

We have constructed a plasmid vector system into which a segment of the Trypanosoma cruzi gene encoding the spliced leader (SL) has been inserted to drive expression of a downstream gene encoding bacterial chloramphenicol acetyltransferase (CAT). We used this construct to establish conditions that permit reproducible transfection of cultured T. cruzi epimastigotes where transfection was mediated by electroporation and measured by assaying expression of CAT. CAT activity was detected only in T. cruzi lysates from cells transfected with constructs containing a properly oriented SL gene; constructs in which the gene was inserted in reverse orientation did not express CAT. The optimal electric field strength, cell density and DNA concentration for efficient transfection were established. This and similar systems will permit genetic dissection of this parasite.

Trypanosoma cruzi strains partition into two groups based on the structure and function of the spliced leader RNA and rRNA gene promoters.

We have previously identified a major proximal sequence element (PSE) responsible for transcription of the spliced leader (SL) gene from Trypanosoma cruzi strain CL, and showed that the sequence encompassing this PSE exhibits approximately 30% divergence between two major groups of T. cruzi isolates, but strong conservation within the groups. In this report, we show that the SL RNA gene promoter from the CL strain (group I) is efficiently expressed only in T. cruzi isolates from group I. Similarly, the sequence of the approximately 643 bp promoter region of the T. cruzi rRNA is strongly conserved within, but diverged approximately 20% between, the two groups. Reporter constructs driven by the rRNA promoter sequences from group I strains are strongly expressed after electroporation into other group I strains, but not expressed in group II strains. In contrast, constructs bearing rRNA promoter sequences from group II strains are active in strains from both groups. Phylogenetic analyses performed with both the rRNA and the SL RNA gene promoter sequences yielded similar trees, and these trees strongly reinforce the partitioning of known T. cruzi into two major groups that parallel the observed functional specificity of the promoters. Given the well-documented species specific pattern of both rRNA promoters and PSEs in higher eukaryotes, these results suggest an ancient evolutionary divergence among organisms currently classified as T. cruzi.

Identification of a spliced leader RNA binding protein from Trypanosoma cruzi.

Nuclear mRNAs in trypanosomatids are generated by trans-splicing. Although trans-splicing resembles cis-splicing in many ways and most of the U RNA participants have been characterized, relatively few involved proteins have been identified. Herein, we employed a yeast three-hybrid system to identify a protein, XB1, which binds to the Trypanosoma cruzi SL RNA. XB1 is a approximately 45 kDa protein which is homologous to the essential pre-mRNA-splicing factor PRP31p from Saccharomyces cerevisiae. Gel shift assays and UV cross-linking experiments with recombinant XB1 confirmed that this T. cruzi protein binds the SL RNA in vitro. The binding site of XB1 on the SL RNA was mapped to stem-loop II by deletion of the SL RNA 'bait' in the three-hybrid system. Finally, UV cross-linking SL RNA with S100 extract indicated native XB1 protein and SL RNA interaction in T. cruzi extract.

PPB1, a putative spliced leader RNA gene transcription factor in Trypanosoma cruzi.

In trypanosomatids, the spliced leader RNA, or SL RNA, donates its 5' 39 nucleotides to mature nuclear mRNAs in a process termed trans-splicing. We have previously characterized the SL RNA gene from Trypanosoma cruzi and identified its transcription promoter, including a 14 nt proximal sequence element, or PSE, that binds a putative transcription factor and activates transcription of the gene. Herein, we describe establishment of a yeast one-hybrid system using the 14 nt PSE as bait, and use this system to select T. cruzi cDNAs encoding a putative transcription factor that activates transcription of the SL RNA gene. The cDNA was selected from a normalized library and encodes an approximately 45 kDa putative PSE promoter-binding protein, PPB1. PPB1 in vitro translated or overexpressed in and isolated from transformed E. coli, showed PSE-specific binding activity by electrophoretic mobility shift assays. Finally, overexpression of PPB1 in T. cruzi led to increased expression of the SL RNA gene as well as reporter genes in episomal constructs under the control of the SL RNA gene promoter. These observations suggest that PPB1 is a transcription factor that plays an important role in SL RNA gene expression.

Transient expression mediated by the Trypanosoma cruzi rRNA promoter.

Plasmid constructs containing a putative Trypanosoma cruzi rRNA promoter and transcription start point upstream from the bacterial chloramphenicol acetyltransferase (CAT) reporter gene were transfected into cultured T. cruzi epimastigotes to verify the presence of a promoter activity. Constructs bearing the putative promoter and a 3' trans-splicing acceptor site in the proper orientation yielded approx. two orders of magnitude greater CAT expression than that previously observed with the T. cruzi spliced leader (SL) gene promoter. In contrast, similar constructs lacking the known 3' splice site yielded reduced but readily measurable expression suggesting that sequences near the promoter may function as cryptic 3' splice sites. A repeated sequence upstream from the putative basal rRNA promoter in a position analogous to rRNA gene enhancer elements in other eukaryotes did not enhance expression from the T. cruzi rRNA promoter. Finally, these constructs were functional in some but not all T. cruzi isolates, and were inactive in other kinetoplastid species, suggesting that the T. cruzi rRNA promoter may have a limited host range.

Animal propagation and genomic survey of a genotype 1 isolate of Cryptosporidium parvum.

Human cryptosporidiosis is attributed to two major Cryptosporidium parvum genotypes of which type 1 appears to be the predominant. Most laboratory investigations however are performed using genotype 2 isolates, the only type which readily infects laboratory animals. So far type 1 has only been identified in humans and primates. A type 1 isolate, obtained from an individual with HIV and cryptosporidiosis, was successfully adapted to propagate in gnotobiotic piglets. Genotypic characterization of oocyst DNA from this isolate using multiple restriction fragment length polymorphisms, a genotype-specific PCR marker, and direct sequence analysis of two polymorphic loci confirmed that this isolate, designated NEMC1, is indeed type 1. No changes in the genetic profile were identified during multiple passages in piglets. In contrast, the time period between infection and onset of fecal oocyst shedding, an indicator of adaptation, decreased with increasing number of passages. Consistent with other type 1 isolates, NEMC1 failed to infect mice. A preliminary survey of the NEMC1 genome covering approximately 2% of the genome and encompassing 200 kb of unique sequence showed an average similarity of approximately 95% between type 1 and 2 sequences. Twenty-four percent of the NEMC1 sequences were homologous to previously determined genotype 2 C. parvum sequences. To our knowledge, this is the first successful serial propagation of genotype 1 in animals, which should facilitate characterization of the unique features of this human pathogen.

Optimization of coupled PCR amplification and cycle sequencing of cloned and genomic DNA.

We describe optimization of a coupled amplification and cycle sequencing (CAS) method for rapid characterization of cloned or genomic DNA. Our modification of this method, termed coupled PCR amplification and cycle sequencing (CPACS), utilizes commercially available reagents, does not require template purification and produces high-quality sequence ladders from nanogram quantities of complex genomic DNA. The reactions have been streamlined to permit automation. Finally, we show that the technique can be applied more efficiently in conjunction with the AutoTrans 350 Direct Transfer Electrophoresis System and 33P-labeled sequencing primers.

A survey of nucleic acid services in core laboratories.

Core facility services related to DNA synthesis and sequencing were surveyed by the Association of Biomolecular Resource Facilities. Responses from 85 facilities offering DNA synthesis and 37 facilities offering DNA sequencing were obtained. Data on instrumentation, volume, number of users, cost, methodology and a number of other criteria were obtained. The volume of work performed by these centralized core facilities was quite substantial (combined synthesis output of 4 million bases per year and a combined sequencing output of 35 million bases per year). The large number of users supported by these facilities and the high sample throughput make these core resource facilities good indicators of technological trends.

Multi-facility survey of oligonucleotide synthesis and an examination of the performance of unpurified primers in automated DNA sequencing.

The purity of 208 crude synthetic 25- and 50-base oligonucleotides synthesized in 71 DNA core facilities was assessed by capillary electrophoresis (CE), and the average coupling efficiency of each synthesis was determined. The median average coupling efficiencies of the 25-mers and 50-mers were 98.9% and 98.7%, respectively, and 85% of the samples exceeded the minimum industry standard of 98% average coupling efficiency. The overall yields estimated by on-line trityl monitors showed poor agreement with the empirically determined yield, and accuracy of the monitors decreased as synthesis efficiency decreased. The performance of the unpurified 25-base oligonucleotides, ranging in purity from 14% to 94%, as primers for automated DNA sequencing was evaluated. Over 85% of these oligonucleotides exhibited an unedited sequencing accuracy of > 97.5% over the 400-base test sequence. Surprisingly, sequencing performance was not strictly related to primer purity, though a marked loss of performance was observed for primers < or = 70% pure (< or = 98.5% coupling efficiency). Thus, the vast majority of the oligonucleotides synthesized by the 71 core facilities participating in this study were of high quality and performed well as sequencing primers without post-synthesis purification or desalting. Finally, our results suggest that an increase in the standard minimum performance specifications of DNA synthesis instruments and reagents from > or = 98% to > or = 98.5% average coupling efficiency, or the development of rapid, inexpensive and efficient methods to detect syntheses below the 98.5% threshold, could obviate post synthesis purification of sequencing primers.

Design strategies and performance of custom DNA sequencing primers.

This study surveyed strategies of sequencing primer selection and evaluated primer performance in automated DNA sequencing. We asked participants to relate their preferred primer design strategies to identify primer characteristics that are considered most important in sequencing primer design. The participants preferred primers of 18-24 nucleotides (nt), 39%-58% G + C, a melting temperature (Tm) of 53 degrees-65 degrees C with a 1-2 nt 3' GC clamp, hairpin stems of less than 2-3 bp, homopolymeric runs of less than 4-5 nt, primer dimers of less than 3-4 bp and secondary priming sites of less than 3-4 bp. We provided a 300-bp test sequence and asked participants to submit sequences of 1-3 optimal sequencing primers. Submitted primers ranged from 17-24 nt and largely conformed to the preferred parameters. Submitted primers were distributed across the test sequence, although some sites were disfavored. Surprisingly, approximately 45% of the primers were selected "manually", more than by any software package. Each of 69 submitted and 95 control primers, distributed at 3-bp intervals across the test sequence, were synthesized, purified and tested using a Model 377 PRISM DNA Sequencer with dichlororhodamine dye terminator reagents (dRhodamine dye terminators). Approximately half of the control primers were also tested using rhodamine dye terminator reagents ("old" rhodamine dye terminators). The results indicated that primer physico-chemical characteristics thought to have a strong impact on sequencing performance had surprisingly little effect. Thus, primers with high or low percent G + C or Tm, strong secondary priming scores or long 3' homopolymeric stretches yielded excellent sequences with the dRhodamine dye terminator reagents, although these characteristics had a stronger effect when the old rhodamine reagents were used. The old rhodamine reagents gave sequences with a similar average read length, but the number of errors and ambiguities or "N's" was consistently higher. Moreover, the effects of the primer physico-chemical characteristics were also more evident with the old rhodamine dyes. We conclude that under optimal sequencing conditions with highly pure template and primer, many of the commonly applied primer design parameters are dispensable, particularly when using one of the new generation of sequencing reagents such as the dichlororhodamine dye terminators.

Accuracy of automated DNA sequencing: a multi-laboratory comparison of sequencing results.

A double-stranded (ds)DNA template of "unknown" sequence was distributed to approximately 80 core DNA sequencing laboratories by the Association of Biomolecular Resource Facilities (ABRF) for automated DNA sequence analysis. Forty-four different facilities responded with 83 usable sequence submissions. These sequences were grouped by both sequencing protocol (dye-primer or dye-terminator) and whether manually edited or not. The sequences were aligned with the known sequence, and the number of correct base calls, insertions, deletions, no-calls and miscalls were determined for each group. The dye-primer sequencing protocol provided the longest and most accurate sequence. The edited dye-primer data were > 95% accurate out to 400-450 bp, while the edited dye-terminator data could call only 300-350 bases at this accuracy. However, 75% of the laboratories in this sampling preferred the dye-terminator protocol, presumably because of its versatility and convenience. Laboratories that manually edited the automatically called data were able to obtain an additional 100 bases of good sequence when the dye-primer protocol was used. Surprisingly though, editing of dye-terminator results did not increase the amount of good sequence, although the dye-terminator protocol had a superior base-calling ability within the first 100 bases of called sequence.

Genetic relatedness among Trypanosoma evansi stocks by random amplification of polymorphic DNA and evaluation of a synapomorphic DNA fragment for species-specific diagnosis.

In this study we employed randomly amplified polymorphic DNA patterns to assess the genetic relatedness among 14 Brazilian Trypanosoma evansi stocks from domestic and wild hosts, which are known to differ in biological characteristics. These akinetoplastic stocks were compared with one another, to three Old World (Ethiopia, China and Philippines) dyskinetoplastic stocks of T. evansi, and also with Trypanosoma equiperdum, Trypanosoma brucei brucei, Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense. Randomly amplified polymorphic DNA analysis showed limited heterogeneity in T. evansi stocks from different hosts and geographical regions of the world, or in other species of the subgenus Trypanozoon. However, minor variations generated random amplification of polymorphic DNA analysis disclosed a pattern consisting of a unique synapomorphic DNA fragment (termed Te664) for the T. evansi cluster that was not detected in any other trypanosome species investigated. Pulsed field gel electrophoresis analysis demonstrated that the Te664 fragment is a repetitive sequence, dispersed in intermediate and minichromosomes of T. evansi. Based on this sequence, we developed a conventional PCR assay for the detection of T. evansi using crude preparations of blood collected either on glass slides or on filter paper as template DNA. Our results showed that this assay may be useful as a diagnostic tool for field-epidemiological studies of T. evansi.

PCR amplification of the spliced leader gene for the diagnosis of trypanosomatid parasites of plants and insects in methanol-fixed smears.

A PCR-based method was adapted for the amplification of DNA from methanol-fixed smears of insects and plants parasitized by trypanosomatids. The PCR target was the multicopy spliced leader (SL) gene. Amplicons were hybridized with an oligonucleotide probe (SL3') specific for Phytomonas. The method has the advantage of dispensing with the cultivation of parasites, many of which are very fastidious or non-cultivable. The technique was applied to archival glass slides and to newly collected material. It proved to specific for Phytomonas spp., enabling their detection in plants and insects. Sequence comparison of the amplicons obtained revealed the existence of different strains/species of Phytomonas circulating among diseased palsms and fruit.

Simultaneous stable expression of neomycin phosphotransferase and green fluorescence protein genes in Trypanosoma cruzi.

The ribosomal RNA (rRNA) gene promoter was used to construct plasmid vectors that simultaneously express multiple exogenous genes in Trypanosoma cruzi. Vector pBSPANEO expresses neomycin phosphotransferase, and pPAGFPAN expresses both green fluorescent protein and neomycin phosphotransferase from a single promoter. Both vectors require the presence of the rRNA promoter for stable transfection; epimastigotes transfected with pPAGFPAN strongly fluoresced due to green fluorescent protein expression. Intact plasmids were rescued from the T. cruzi-transfected population after >8 mo of culture, indicating stable replication of these vectors. Vectors were integrated into the rRNA locus by homologous recombination and into other loci, presumably by illegitimate recombination. Parasites bearing tandem concatamers of plasmids were also found among the transfectants. Transfectants expressing green fluorescent protein showed a bright green fluorescence distributed throughout the cell. Fluorescence was also detected in amastigotes after infection of mammalian cells with transfected parasites, indicating that the rRNA promoter can drive efficient expression of these reporter genes in multiple life-cycle stages of the parasite. Expression of the heterologous genes was detected after passage in mice or in the insect vector. These vectors will be useful for the genetic dissection of T. cruzi biology and pathogenesis.