SERINE HYDROXYMETHYLTRANSFERASE IS MATERNALLY ESSENTIAL IN CAENORHABDITIS-ELEGANS

The mel-32 gene in the free living soil nematode Caenorhabditis elegan s encodes a serine hydroxymethyltransferase (SHMT) isoform, Seventeen ethylmethanesulfonate (EMS)-induced mutant alleles of mel-32(SHMT) hav e been generated, each of which causes a recessive maternal effect let hal phenotype. Animals homozygous for the SHMT mutations have no obser vable mutant phenotype, but their offspring display an embryonic letha l phenotype, The Mel-32 phenotype has been rescued with a transgenic a rray containing only mel-32(SHMT) genomic DNA, Heteroduplex analysis o f the 17 alleles allowed 14 of the mutations to be positioned to small regions, Subsequent sequence analysis has shown that 16 of the allele s alter highly conserved amino acids, while one allele introduces a st op codon that truncates two thirds of the predicted protein, mel-32(SH MT) has a 55-60% identity at the amino acid level with both isoforms o f SHMT found in yeast and humans and a 50% identity with the Escherich ia coli isoform, The C, elegans mel-32 mutation represents the first c ase where SHMT has been shown to be an essential gene.