There is likely a functional significance for the duplication of metabolic genes in the genome of a parasite that has to convert between developmental stages under different micro-environmental

conditions during the asexual phase of its life cycle. It has been suggested, for instance, that stage-specific expression of different isoforms of metabolic enzymes such as lactate dehydrogenase and enolase (ENO) may be reflective of the different metabolic states of tachyzoites and bradyzoites, with tachyzoites being the more metabolically active. This assertion is supported by the fact that recombinant tachyzoite-specific enolase 2 (ENO2) displays higher activity in vitro than the bradyzoite-specific ENO1 (28–30). The differential expression of isoforms with varying activity levels between the two developmental stages is therefore consistent BAY 73-4506 order with their respective metabolic requirements (28). Ferguson et al. provide an alternate view highlighting the fact that early bradyzoites are just as metabolically active as tachyzoites and that the expression of bradyzoite-specific metabolic enzymes might be a feature that is adaptive to the different growth conditions

encountered by these developmental stages, with varying resource constraints (31,32). In another genome-wide search, the complement of genes encoding enzymes involved in metabolism of amylopectin has been identified in the Toxoplasma genome (33). It Fluorouracil is interesting to note that some of these genes also exhibit stage-specific expression profiles. R1 protein, α-glucan phosphorylase, α-glucosidase and α-amylase, which perform catabolic functions, NVP-AUY922 clinical trial are preferentially expressed

in bradyzoites. On the other hand, enzymes involved in synthesis such as glycogenin, glycogen synthase and branching enzyme are predominantly expressed in tachyzoites (33). This expression pattern is consistent with the observation of amylopectin accumulation and subsequent turnover during differentiation (33,34). The use of microarrays in Toxoplasma studies has allowed for genome-wide queries of gene expression patterns and other genome-wide association studies that have had a significant impact on our understanding of the parasite’s biology. The first generation of Toxoplasma microarrays was designed to be used in the study of differential gene expression between the tachyzoite and bradyzoite stages of the asexual cycle (35). This array was constructed from a bradyzoite cDNA library, which represented a minimum of 600 genes. cDNAs were spotted onto glass slides and used to probe gene transcripts isolated from tachyzoites or bradyzoites. In spite of the inherent limitation of these arrays in terms of gene coverage (600 of approximately 8000 predicted genes), they have been very useful in identifying stage-specific genes that have proven to be important in differentiation (35–37).