Serine hydroxymethyltransferase (SHMT) is a ubiquitous enzyme involved in one carbon metabolism. In converting serine to glycine it transfers a methyl group to tetrahydrofolate and, hence, into the one carbon pool. This tetrahydrofolate pool is utilized during the biosynthesis of thymidine, lipids, and other essential molecules. A computer homology search for serine hydroxymethyltransferase homologs in C. elegans revealed one gene, C05D11.11. The amino acid sequence of this enzyme is highly conserved, and the C. elegans form has ~60% identity with the eukaryotic homologs. The cosmid C05D11 rescued five essential genes in the region, while a subclone containing CeSHMT by itself rescued the maternal effect lethal phenotype of
mel-32. (See abstract by G. Vatcher) The C. elegans mutant
mel-32 represents the only known instance where a SHMT mutant is lethal. There are 17 alleles of
mel-32 and all have essentially the same terminal phenotype, F2 egg arrest between the 56-100 cell stage. Mutant embryos show a delayed cell cycle, possibly consistent with a lack of thymidine. Heteroduplex analysis of the 17 alleles with 5 overlapping PCR fragments has positioned 13 of the mutations into specific regions of the gene. Sequence analysis of the specific regions of each mutant is underway. It is hoped that the availability of these 17 alleles will help determine which of the highly conserved residues is essential to SHMT function. The intragenic complementation tests of these alleles shows a complex pattern, with some combinations surviving until early larval stages. This pattern suggests that SHMT is a multimeric protein. SHMT levels are increased in tumor cells and rapidly proliferating cell lines and this enzyme is a potential chemotherapy target. It is hoped that the work done on this enzyme in C. elegans will have relevance in the search for new anti-cancer agents. This work was supported by a grant from the Medical Research Council of Canada to Ann Rose and Dave Baillie.