MUTATIONS IN GENES ENCODING EXTRACELLULAR-MATRIX PROTEINS SUPPRESS THE EMB-5 GASTRULATION DEFECT IN CAENORHABDITIS-ELEGANS

The second division of the gut precursor E cells is lethally accelerat ed during Caenorhabditis elegans gastrulation by mutations in the emb- 5 gene, which encodes a presumed nuclear protein. We have isolated sup pressor mutations of the temperature-sensitive allele emb-5(hc61), scr eened for them among dpy and other mutations routinely used as genetic markers, and identified eight emb-5 suppressor genes. Of these eight suppressor genes, at least four encode extracellular matrix proteins, i.e., three collagens and one proteoglycan. The suppression of the emb -5 gastrulation defect seemed to require the maternal expression of th e suppressors. Phenotypically, the suppressors by themselves slowed do wn early embryonic cell divisions and corrected the abnormal cell-divi sion sequence of emb-5 mutant embryos, We propose an indirect stress-r esponse mechanism to be the main cause of the suppression because: (1) none of these suppressors is specific, either to particular temperatu re-sensitive emb-5 alleles or to the emb-5 gene; (2) suppressible alle les of genes, reported here or elsewhere, are temperature sensitive or weak; (3) the suppression is not Strong but marginal (4) the suppress ion itself shows some degree of temperature dependency; and (5) none o f the extracellular matrix proteins identified here is known to be exp ressed in oocytes or early embryos, despite the present observation th at the suppression is maternal.