Molecular cloning and characterization of the genes encoding two isoforms of cysteine synthase in the enteric protozoan parasite Entamoeba histolytica

The enteric protozoan parasite Entamoeba histolytica was shown to possess c ysteine synthase (CS) activity. The cDNA and genomic clones that encode two isoforms of the E. histolytica CS were isolated and characterized from a c lonal strain of E. histolytica by genetic complementation of the cysteine-a uxotrophic Escherichia coli NK3 with an E. histolytica cDNA library. The tw o types of the E, histolytica CS genes differed from each other by three nu cleotides, two of which resulted in amino acid substitution. Deduced amino acid sequences of the E. histolytica CS, with a calculated molecular mass o f 36 721 Da and an isoelectric point of 6.39, exhibited 38-48% identity wit h CS of bacterial and plant origins. The absence of the amino-terminal tran sit peptide in the deduced protein sequences and the presence of the CS pro tein mainly in the supernatant fraction of the amoebic lysate after cellula r fractionation suggested that the identified E. histolytica CS genes encod ed cytosolic isoforms. Substrate specificity of the recombinant E. histolyt ica CS was similar to that of plant CS. Phylogenetic analysis indicates tha t the amoebic CS, first described in Protozoa, does not belong to any famil ies of the CS superfamily, and represents a new family. (C) 1998 Elsevier S cience B.V. All rights reserved.