Etylation of the side chain hydroxyl group of L-serine to form O-acetylserine (OAS) [13]. Subsequently, cysteine synthase [(CS; OAS (thiol) lyase; EC4.2.99.8)] catalyzes the reaction of OAS with methanethiol or sulfide to produce WP1066 site S-methylcysteine or L-cysteine, respectively. Recombinant amebic CS isotypes possess both S-methylcysteine and L-cysteine synthesizing activities in vitro. However, our recent in vivo study [12] revealed that CS isotypes are primarily involved in the synthesis of SMC, but not of Lcysteine. Since, this pathway is not involved in the synthesis of L-cysteine, in vitro cultivation of amebic trophozoites requires high concentrations of Lcysteine, and this requirement can not be replaced by other thiols [14]. In E. histolytica, L-cysteine is required for the growth, attachment, survival, and protection from oxidative stress [14,15]. All prokaryotic and eukaryotic cells are known to have an ability to restructure their transcriptomes in order to adapt to the environmental conditions by sensing the endogenous level of various metabolites. Small-molecule metabolites, including amino acids, nucleotides, and carbohydrates have been shown to regulate the expression of large number of genes at the transcriptional and post-transcriptional levels [16]. In addition, intracellular redox determined by various metabolites has also been demonstrated to be an important regulator to gene expression [16]. In most eukaryotes, glutathione is the major thiol, and L-cysteine levels are maintained many fold lower than that of glutathione [17]. However, E. histolytica completely lacks glutathione metabolism and relies on L-cysteine as a major redox buffer [5, 6, and 8]. Therefore, E. histolytica represents an excellent model to study the effect of L-cysteine deprivation on gene expression and cellular metabolism. Our recent metabolomic study PubMed ID: demonstrated that in E. histolytica, L-cysteine regulates various metabolic pathways, including energy, amino acid, and phospholipid metabolism [12]. In this study we performed DNA microarray analysis of gene expression in E. histolytica cultured in L-cysteine-deprived conditions. We found that the expression of a large number of genes was modulated in response to the Lcysteine deprivation.Results and DiscussionsL-Cysteine deprivation induces global changes in the gene expressionTo better understand the role of L-cysteine in transcriptional regulation of gene expression in E. histolytica, we performed time course analysis of genome wide gene expression upon L-cysteine deprivation, using a custom-made Affymetrix microarray representing 9,327 of E. histolytica genes. We identified 290 genes (3.1 ) modulated by at least 3 fold (p-value < 0.05) at one or more time points in response to Lcysteine deprivation (Additional file 1). Out of them, 129 genes were up-regulated and 167 genes were down-regulated, while 6 genes showed both up- and down-regulation depending upon the time points (Tables 1 and 2; Additional files 2 and 3). Out of the 129 up-regulated genes, 51 genes (40 ) were assigned with putative biological functions, namely signalling, general metabolism, lipid metabolism, DNA/RNA regulation, electron transport, stress response, transport, and trafficking/secretion/cytoskeleton (Figure 1). The remaining 78 genes (60 ) were categorized into genes encoding either hypothetical proteins without (68) or with known conserved domain(s) (10). A total of 167 genes were down regulated by 3 fold at one or more.