Deficiency in the Essential Amino Acids l-Isoleucine, l-Leucine and l-Histidine and Clinical Measures as Predictors of Moderate Depression in Elderly Women: A Discriminant Analysis Study.

Increases in depression are common in some elderly women. Elderly women often show moderate depressive symptoms, while others display minimal depressive symptoms. These discrepancies have produced contradictory and inconclusive outcomes, which have not been explained entirely by deficits in neurotransmitter precursors. Deficiency in some amino acids have been implicated in major depression, but its role in non-clinical elderly women is not well known. An analysis of essential amino acids, depression and the use of discriminant analysis can help to clarify the variation in depressive symptoms exhibited by some elderly women. The aim was to investigate the relationship of essential amino acids with affective, cognitive and comorbidity measures in elderly women without major depression nor severe mood disorders or psychosis, specifically thirty-six with moderate depressive symptoms and seventy-one with minimal depressive symptoms. The plasma concentrations of nineteen amino acids, Beck Depression Inventory (BDI) scores, Geriatric Depression Scale (GDS) scores, global cognitive scores and comorbidities were submitted to stepwise discriminant analysis to identify predictor variables. Seven predictors arose as important for belong to the group based on amino acid concentrations, with the moderate depressive symptoms group characterized by higher BDI, GDS and cognitive scores; fewer comorbidities; and lower levels of l-histidine, l-isoleucine and l-leucine. These findings suggest that elderly women classified as having moderate depressive symptoms displayed a deficiency in essential amino acids involved in metabolism, protein synthesis, inflammation and neurotransmission.

The response of Plasmodium falciparum to isoleucine withdrawal is dependent on the stage of progression through the intraerythrocytic cell cycle.

BACKGROUND: A previous study reported that the malaria parasite Plasmodium falciparum enters an altered growth state upon extracellular withdrawal of the essential amino acid isoleucine. Parasites slowed transit through the cell cycle when deprived of isoleucine prior to the onset of S-phase. METHODS: This project was undertaken to study at higher resolution, how isoleucine withdrawal affects parasite growth. Parasites were followed at regular intervals across an extended isoleucine deprivation time course across the cell cycle using flow cytometry. RESULTS: These experiments revealed that isoleucine-deprived parasites never exit the cell cycle, but instead continuously grow at a markedly reduced pace. Moreover, slow growth occurs only if isoleucine is removed prior to the onset of schizogony. After S-phase commenced, the parasite is insensitive to isoleucine depletion and transits through the cell cycle at the normal pace. CONCLUSIONS: The markedly different response of the parasite to isoleucine withdrawal before or after the onset of DNA replication is reminiscent of the nutrient-dependent G1 cell cycle checkpoints described in other organisms.

Effects of medical food leucine content in the management of methylmalonic and propionic acidemias.

PURPOSE OF REVIEW: The current review highlights the varied effects of medical foods high in leucine (Leu) and devoid of valine (Val) and isoleucine (Ile) in the management of methylmalonic acidemia (MMA) and propionic acidemia and cobalamin C (cblC) deficiency, aiming to advance dietary practices. RECENT FINDINGS: Leu is a key metabolic regulator with a multitude of effects on different organ systems. Recent observational studies have demonstrated that these effects can have unintended consequences in patients with MMA as a result of liberal use of medical foods. The combination of protein restriction and medical food use in MMA and propionic acidemia results in an imbalanced branched-chain amino acid (BCAA) dietary content with a high Leu-to-Val and/or Ile ratio. This leads to decreased plasma levels of Val and Ile and predicts impaired brain uptake of multiple essential amino acids. Decreased transport of methionine (Met) across the blood-brain barrier due to high circulating Leu levels is of particular concern in cblC deficiency in which endogenous Met synthesis is impaired. SUMMARY: Investigations into the optimal composition of medical foods for MMA and propionic acidemia, and potential scenarios in which Leu supplementation may be beneficial are needed. Until then, MMA/propionic acidemia medical foods should be used judiciously in the dietary management of these patients and avoided altogether in cblC deficiency.

Gill structural integrity changes in fish deficient or excessive in dietary isoleucine: Towards the modulation of tight junction protein, inflammation, apoptosis and antioxidant defense via NF-κB, TOR and Nrf2 signaling pathways.

This study firstly aimed to test the impact of dietary isoleucine (Ile) on tight junction protein, inflammation, apoptosis, antioxidant defense and related signaling molecule gene expression in the gill of fish. Young grass carp (Ctenopharyngodon idella) (weighing 256.8 ± 3.5 g) were fed six diets containing graded levels of Ile, namely, 3.8, 6.6, 9.3, 12.5, 15.2 and 18.5 g/kg diet for 8 weeks. The results firstly revealed that Ile deficiency down-regulated the mRNA expressions of claudin-3, claudin-b, claudin-c, occludin and zonula occludens-1 (ZO-1) and up-regulated the mRNA expression of claudin-12, which led to the intercellular structure damage of fish gill. These effects were partially ascribed to the up-regulation of pro-inflammatory cytokines [interleukin 1ß (IL-1ß), interleukin 8 (IL-8) and tumor necrosis factor-α (TNF-α)] mRNA expressions that referring to up-regulated nuclear factor κB P65 (NF-κB P65) mRNA expression and down-regulated inhibitor factor κBα (IκBα) mRNA expression, and the down-regulation of anti-inflammatory cytokines [interleukin 10 (IL-10) and transforming growth factor ß1 (TGF-ß1)] mRNA expressions that referring to the down-regulated TOR and S6K1 mRNA expression. Interestingly, no change in claudin 15 mRNA level was observed among every treatment. At the same time, the results firstly indicated that Ile deficiency also resulted in the cellular structure damage of fish gill: (1) DNA fragmentation partially due to the up-regulation of caspase-3, caspase-8 and caspase-9 mRNA expression; (2) increase in protein carbonyl (PC), malondialdehyde (MDA) and ROS contents, which may be partially attributed to the impaired antioxidant defense [indicated by decreased glutathione (GSH) level and depressed anti-superoxide anion (ASA), anti-hydroxyl radical (a-HR), copper/zinc superoxide dismutase (Cu/Zn-SOD), catalase (CAT) and glutathione peroxidase (GPx) activities] that referring to the down-regulation of corresponding antioxidant enzyme mRNA expressions and the related signaling molecules Nrf2 mRNA expression. Ile excess caused similar negative effects that observed in Ile-deficient group, whereas these negative effects were reversed with appropriate Ile supplementation. In conclusion, our results indicated that Ile deficiency or excess disrupted the structural integrity of fish gill, partially due to the trigger of apoptosis, the impairment of antioxidant defense, and the regulation of tight junction protein, inflammatory cytokines, apoptosis-related, antioxidant enzymes and related signaling molecules mRNA expressions in the fish gill.

Isoleucine Deficiency in a Neonate Treated for Maple Syrup Urine Disease Masquerading as Acrodermatitis Enteropathica.

BACKGROUND: Special diet with restricted branched-chain-amino-acids used for treating maple syrup urine disease can lead to specific amino acid deficiencies. CASE CHARACTERISTICS: We report a neonate who developed skin lesions due to isoleucine deficiency while using specialised formula. INTERVENTION/OUTCOME: Feeds were supplemented with expressed breast milk. This caused biochemical and clinical improvement with resolution of skin lesions. MESSAGE: Breast milk is a valuable and necessary adjunct to specialized formula in maple syrup urine disease to prevent specific amino acid deficiency in the neonatal period.

Acrodermatitis dysmetabolica in an infant with maple syrup urine disease.

Acrodermatitis dysmetabolica (AD) is a rare, newly termed, and poorly understood disease that appears to be clinically similar to acrodermatitis enteropathica (AE). Both diseases are characterized by the triad of periorificial and acral dermatitis, diarrhoea, and alopecia. Unlike AE, which is caused by zinc deficiency, AD is caused by numerous metabolic disorders. One such disorder is maple syrup urine disease (MSUD), a genetic deficiency of branched chain α-ketoacid dehydrogenase, the enzyme that degrades the branched-chain amino acids (BCAAs) isoleucine, leucine and valine. Treatment involves restricting BCAAs to prevent accumulation. We report a case of an infant being treated for MSUD, who developed the triad of AE/AD after a period of poor BCAA formula intake. The child was found to have low isoleucine and normal zinc levels. Increasing the isoleucine dose improved the eruption, thus the diagnosis of AD secondary to isoleucine deficiency was made. This case emphasizes the importance of carefully balancing BCAA levels while treating MSUD, as deficiency can precipitate AD.

Effects of individual branched-chain amino acids deprivation on insulin sensitivity and glucose metabolism in mice.

OBJECTIVE: We recently discovered that leucine deprivation increases hepatic insulin sensitivity via general control nondepressible (GCN) 2/mammalian target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK) pathways. The goal of the present study was to investigate whether the above effects were leucine specific or were also induced by deficiency of other branched chain amino acids including valine and isoleucine. METHODS: Following depletion of BCAAs, changes in metabolic parameters and the expression of genes and proteins involved in regulation of insulin sensitivity and glucose metabolism were analyzed in mice and cell lines including human HepG2 cells, primary mouse hepatocytes and a mouse myoblast cell line C2C12. RESULTS: Valine or isoleucine deprivation for 7 days has similar effect on improving insulin sensitivity as leucine, in wild type and insulin-resistant mice models. These effects are possibly mediated by decreased mTOR/S6K1 and increased AMPK signaling pathways, in a GCN2-dependent manner. Similar observations were obtained in in vitro studies. In contrast to leucine withdrawal, valine or isoleucine deprivation for 7 days significantly decreased fed blood glucose levels, possibly due to reduced expression of a key gluconeogenesis gene, glucose-6-phosphatase. Finally, insulin sensitivity was rapidly improved in mice 1 day following maintenance on a diet deficient for any individual BCAAs. CONCLUSIONS: Our results show that while improvement on insulin sensitivity is a general feature of BCAAs depletion, individual BCAAs have specific effects on metabolic pathways, including those that regulate glucose level. These observations provide a conceptual framework for delineating the molecular mechanisms that underlie amino acid regulation of insulin sensitivity.

The significance of translation regulation in the stress response.

BACKGROUND: The stress response in bacteria involves the multistage control of gene expression but is not entirely understood. To identify the translational response of bacteria in stress conditions and assess its contribution to the regulation of gene expression, the translational states of all mRNAs were compared under optimal growth condition and during nutrient (isoleucine) starvation. RESULTS: A genome-scale study of the translational response to nutritional limitation was performed in the model bacterium Lactococcus lactis. Two measures were used to assess the translational status of each individual mRNA: the fraction engaged in translation (ribosome occupancy) and ribosome density (number of ribosomes per 100 nucleotides). Under isoleucine starvation, half of the mRNAs considered were translationally down-regulated mainly due to decreased ribosome density. This pattern concerned genes involved in growth-related functions such as translation, transcription, and the metabolism of fatty acids, phospholipids and bases, contributing to the slowdown of growth. Only 4% of the mRNAs were translationally up-regulated, mostly related to prophagic expression in response to stress. The remaining genes exhibited antagonistic regulations of the two markers of translation. Ribosome occupancy increased significantly for all the genes involved in the biosynthesis of isoleucine, although their ribosome density had decreased. The results revealed complex translational regulation of this pathway, essential to cope with isoleucine starvation.To elucidate the regulation of global gene expression more generally, translational regulation was compared to transcriptional regulation under isoleucine starvation and to other post-transcriptional regulations related to mRNA degradation and mRNA dilution by growth. Translational regulation appeared to accentuate the effects of transcriptional changes for down-regulated growth-related functions under isoleucine starvation although mRNA stabilization and lower dilution by growth counterbalanced this effect. CONCLUSIONS: We show that the contribution of translational regulation to the control of gene expression is significant in the stress response. Post-transcriptional regulation is complex and not systematically co-directional with transcription regulation. Post-transcriptional regulation is important to the understanding of gene expression control.

Partial deficiency of isoleucine impairs root development and alters transcript levels of the genes involved in branched-chain amino acid and glucosinolate metabolism in Arabidopsis.

Isoleucine is one of the branched-chain amino acids (BCAAs) that are essential substrates for protein synthesis in all organisms. Although the metabolic pathway for isoleucine has been well characterized in higher plants, it is not known whether it plays a specific role in plant development. In this study, an Arabidopsis mutant, lib (low isoleucine biosynthesis), that has defects in both cell proliferation and cell expansion processes during root development, was characterized. The lib mutant carries a T-DNA insertion in the last exon of the OMR1 gene that encodes a threonine deaminase/dehydratase (TD). TD catalyses the deamination and dehydration of threonine, which is the first and also the committed step in the biosynthesis of isoleucine. This T-DNA insertion results in a partial deficiency of isoleucine in lib root tissues but it does not affect its total protein content. Application of exogenous isoleucine or introduction of a wild-type OMR1 gene into the lib mutant can completely rescue the mutant phenotypes. These results reveal an important role for isoleucine in plant development. In addition, microarray analysis indicated that the partial deficiency of isoleucine in the lib mutant triggers a decrease in transcript levels of the genes encoding the major enzymes involved in the BCAA degradation pathway; the analysis also indicated that many genes involved in the biosynthesis of methionine-derived glucosinolates are up-regulated.

Plasmodium falciparum responds to amino acid starvation by entering into a hibernatory state.

The human malaria parasite Plasmodium falciparum is auxotrophic for most amino acids. Its amino acid needs are met largely through the degradation of host erythrocyte hemoglobin; however the parasite must acquire isoleucine exogenously, because this amino acid is not present in adult human hemoglobin. We report that when isoleucine is withdrawn from the culture medium of intraerythrocytic P. falciparum, the parasite slows its metabolism and progresses through its developmental cycle at a reduced rate. Isoleucine-starved parasites remain viable for 72 h and resume rapid growth upon resupplementation. Protein degradation during starvation is important for maintenance of this hibernatory state. Microarray analysis of starved parasites revealed a 60% decrease in the rate of progression through the normal transcriptional program but no other apparent stress response. Plasmodium parasites do not possess a TOR nutrient-sensing pathway and have only a rudimentary amino acid starvation-sensing eukaryotic initiation factor 2α (eIF2α) stress response. Isoleucine deprivation results in GCN2-mediated phosphorylation of eIF2α, but kinase-knockout clones still are able to hibernate and recover, indicating that this pathway does not directly promote survival during isoleucine starvation. We conclude that P. falciparum, in the absence of canonical eukaryotic nutrient stress-response pathways, can cope with an inconsistent bloodstream amino acid supply by hibernating and waiting for more nutrient to be provided.

Isoleucine and leucine independently regulate mTOR signaling and protein synthesis in MAC-T cells and bovine mammary tissue slices.

Understanding the regulatory effects of individual amino acids (AA) on milk protein synthesis rates is important for improving protein and AA requirement models for lactation. The objective of this study was to examine the effects of individual essential AA (EAA) on cellular signaling and fractional protein synthesis rates (FSR) in bovine mammary cells. Omission of L-arginine, L-isoleucine, L-leucine, or all EAA reduced (P < 0.05) mammalian target of rapamycin (mTOR; Ser2448) and ribosomal protein S6 (rpS6; Ser235/236) phosphorylation in MAC-T cells. Phosphorylation of mTOR and rpS6 kinase 1 (S6K1; Thr389) decreased (P < 0.05) in the absence of L-isoleucine, L-leucine, or all EAA in lactogenic mammary tissue slices. Omission of L-tryptophan also reduced S6K1 phosphorylation (P = 0.01). Supplementation of L-leucine to media depleted of EAA increased mTOR and rpS6 and decreased eukaryotic elongation factor 2 (Thr56) phosphorylation (P < 0.05) in MAC-T cells. Supplementation of L-isoleucine increased mTOR, S6K1, and rpS6 phosphorylation (P < 0.05). No single EAA considerably affected eukaryotic initiation factor 2-α (eIF2α; Ser51) phosphorylation, but phosphorylation was reduced in response to provision of all EAA (P < 0.04). FSR declined when L-isoleucine (P = 0.01), L-leucine (P = 0.01), L-methionine (P = 0.02), or L-threonine (P = 0.07) was depleted in media and was positively correlated (R = 0.64, P < 0.01) with phosphorylation of mTOR and negatively correlated (R = -0.42, P = 0.01) with phosphorylation of eIF2α. Such regulation of protein synthesis will result in variable efficiency of transfer of absorbed EAA to milk protein and is incompatible with the assumption that a single nutrient limits protein synthesis that is encoded in current diet formulation strategies.

Isoleucine or valine deprivation stimulates fat loss via increasing energy expenditure and regulating lipid metabolism in WAT.

There has been a growing interest in controlling body weight by increasing dietary levels of leucine recently. By contrast, we have focused on studying the effect of deficiency of branched-chain amino acids (BCAAs) leucine on lipid metabolism. We previously have shown that mice fed a leucine-deficient diet for 7 days exhibit significant changes in lipid metabolism as demonstrated by suppressed lipogenesis in the liver and increased fat mobilization in white adipose tissue, the latter of which was found to be caused by increased lipolysis in WAT and uncoupling protein 1 expression in brown adipose tissue. The goal of our current study is to investigate whether the above effects of leucine deficiency can be generalized to the deficiency of other BCAAs including valine and isoleucine. In our current study, we show that valine or isoleucine deficiency has similar effects on reducing fat mass to leucine deprivation, in a similar manner as those observed during leucine deprivation.

Branched-chain amino acids in liver disease: new aspects of long known phenomena.

PURPOSE OF REVIEW: To provide an overview of findings on the role of branched-chain amino acids (BCAAs) in the pathophysiology, pathobiochemistry, and treatment of liver cirrhosis and its complications that have been published since or were not included in the last review on this topic in 2007 in this journal. RECENT FINDINGS: There has been continued interest in the potential of oral BCAA supplements in improving energy metabolism, nitrogen metabolism, carbohydrate metabolism, insulin resistance, severity of liver disease, serum albumin levels, quality of serum albumin, or postoperative complication rates. Unfortunately, many trials suffer from lacking or inadequate controls or small sample size. In a fine example of scientific perseverance, Dutch researchers uncovered the long-known phenomenon of ingested blood being highly comagenic in patients with cirrhosis to be due to the low biologic value of blood protein. The absence of isoleucine and the abundance of leucine in the hemoglobin molecule by way of BCAA antagonism leads to impaired protein synthesis and azotemia paving the way for hepatic encephalopathy. SUMMARY: Recognizing hypoisoleucinemia and BCAA antagonism following gastrointestinal bleeding, and its successful treatment by isoleucine infusion has advanced our understanding of the role of BCAA in liver cirrhosis.

Calmodulin-androgen receptor (AR) interaction: calcium-dependent, calpain-mediated breakdown of AR in LNCaP prostate cancer cells.

Chemotherapy of prostate cancer targets androgen receptor (AR) by androgen ablation or antiandrogens, but unfortunately, it is not curative. Our attack on prostate cancer envisions the proteolytic elimination of AR, which requires a fuller understanding of AR turnover. We showed previously that calmodulin (CaM) binds to AR with important consequences for AR stability and function. To examine the involvement of Ca(2+)/CaM in the proteolytic breakdown of AR, we analyzed LNCaP cell extracts that bind to a CaM affinity column for the presence of low molecular weight forms of AR (intact AR size, approximately 114 kDa). Using an antibody directed against the NH(2)-terminal domain (ATD) of AR on Western blots, we identified approximately 76-kDa, approximately 50-kDa, and 34/31-kDa polypeptides in eluates of CaM affinity columns, suggesting the presence of CaM-binding sites within the 31/34-kDa ATD of AR. Under cell-free conditions in the presence of phenylmethylsulfonyl fluoride, AR underwent Ca(2+)-dependent degradation. AR degradation was inhibited by N-acetyl-leu-leu-norleu, an inhibitor of thiol proteases, suggesting the involvement of calpain. In intact cells, AR breakdown was accelerated by raising intracellular Ca(2+) using calcimycin, and increased AR breakdown was reversed with the cell-permeable Ca(2+) chelator bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetra-(acetoxymethyl)-ester. In CaM affinity chromatography studies, the Ca(2+)-dependent protease calpain was bound to and eluted from the CaM-agarose column along with AR. Caspase-3, which plays a role in AR turnover under stress conditions, did not bind to the CaM column and was present in the proenzyme form. Similarly, AR immunoprecipitates prepared from whole-cell extracts of exponentially growing LNCaP cells contained both calpain and calpastatin. Nuclear levels of calpain and calpastatin (its endogenous inhibitor) changed in a reciprocal fashion as synchronized LNCaP cells progressed from G(1) to S phase. These reciprocal changes correlated with changes in AR level, which increased in late G(1) phase and decreased as S phase progressed. Taken together, these observations suggest potential involvement of AR-bound CaM in calcium-controlled, calpain-mediated breakdown of AR in prostate cancer cells.

[Acrodermatitis enteropathica-like syndrome secondary to branched-chain amino acid deficiency during treatment of maple syrup urine disease]. / Pseudo-acrodermatite entéropathique secondaire à un déficit en acides aminés ramifiés au cours du traitement d'une leucinose.

INTRODUCTION: Clinical pictures resembling acrodermatitis enteropathica have been described in acquired zinc deficiency and deficiencies of other nutrients such as biotin, essential fatty acids and amino acids as well as biotin metabolism disorders. We describe the case of an infant with maple syrup urine disease who developed an acrodermatitis-like syndrome due to iatrogenic valine and isoleucine deficiency. CASE-REPORT: A diagnosis of maple syrup urine disease was made in a 5-month-old infant girl with severe neurologic disorders with extremely high levels of the three branched-chain amino acids (leucine, valine and isoleucine) in plasma and urine. Seven days after the start of therapy with a diet excluding these branched-chain amino acids, plasma isoleucine and valine concentrations were low while plasma leucine remained elevated. At the same time, a periorificial and acral dermatitis appeared together with diarrhea. Serum zinc concentrations were normal. A diagnosis of acrodermatitis enteropathica-like syndrome secondary to isoleucine and valine deficiency was suspected. Valine and isoleucine supplementation resulted in rapid resolution of the eruption. DISCUSSION: Several cases of acrodermatitis enteropathica-like eruptions resulting from therapeutic protein restriction diets have been described in infants with different aminoacidopathies. The accompanying dermatosis was associated with a raised plasma leucine/isoleucine ratio and/or isoleucine deficiency, or valine deficiency. While the exact pathogenesis of the skin lesions has not been established, these observations show that branched-chain amino acids are essential for normal growth and differentiation of keratinocytes. The essential role of isoleucine is further substantiated by the fact that its presence is critical in keratinocyte culture media, with growth arrest occurring upon its depletion.

[Iatrogenic acrodermatitis enteropathica-like syndrome in leucinosis]. / Pseudo-acrodermatite entéropathique per carence alimentaire en isoleucine chez un nourrisson atteint de leucinose.

INTRODUCTION: Leucinosis (maple syrup urine disease) is a metabolic disorder caused by an enzymatic deficiency involved in the degradative pathways of the three branched-chain amino acids. We report an observation of acrodermatitis enteropathica-like syndrome induced by essential amino acid deficiency in a child with leucinosis. CASE REPORT: A child with leucinosis was referred to our hospital for exfoliative dermatitis of the perioral and anogenital regions associated with diarrhea and pancytopenia. The diagnosis of iatrogenic acrodermatitis enteropathica-like syndrome was confirmed after screening showing isoleucine deficiency. Rapid response was observed after adequate isoleucine supplementation. DISCUSSION: The acrodermatitis enteropathica-like eruption in our patient was due to an iatrogenic amino acid nutritional imbalance. Our observation underlines the risk of using a branched-chain amino acid-free formula without adequate supplementation of deficient amino acids. In addition, dietary insufficiency of isoleucine, associated with the treatment of organic aciduria should be added to the causes of acrodermatitis enteropathica-like syndrome.

Recruitment of the ArgR/Mcm1p repressor is stimulated by the activator Gcn4p: a self-checking activation mechanism.

Transcription of the arginine biosynthetic gene ARG1 is repressed by the ArgR/Mcm1p complex in arginine-replete cells and activated by Gcn4p, a transcription factor induced by starvation for any amino acid. We show that all four subunits of the arginine repressor are recruited to ARG1 by Gcn4p in cells replete with arginine but starved for isoleucine/valine. None of these proteins is recruited to the Gcn4p target genes ARG4 and SNZ1, which are not regulated by ArgR/Mcm1p. Mcm1p and Arg80p were found in a soluble complex lacking Arg81p and Arg82p, and both Mcm1p and Arg80p were efficiently recruited to ARG1 in wild-type cells in the presence or absence of exogenous arginine, and also in arg81Delta cells. By contrast, the recruitment of Arg81p and Arg82p was stimulated by exogenous arginine. These findings suggest that Gcn4p constitutively recruits an Mcm1p/Arg80p heterodimer and that efficient assembly of a functional repressor also containing Arg81p and Arg82p occurs only in arginine excess. By recruiting an arginine-regulated repressor, Gcn4p can precisely modulate its activation function at ARG1 according to the availability of arginine.