IDENTIFICATION OF HETEROCHRONIC MUTANTS IN CAENORHABDITIS-ELEGANS - TEMPORAL MISEXPRESSION OF A COLLAGEN /GREEN FLUORESCENT PROTEIN FUSION GENE/

The heterochronic genes lin-4, lin-14, lin-28, and lin-29 specify the timing of lateral hypodermal seam cell terminal differentiation in Cae norhabditis elegans. We devised a screen to identify additional genes involved in this developmental timing mechanism based on identificatio n of mutants that exhibit temporal misexpression from the col-19 promo ter, a downstream target of the heterochronic gene pathway. We fused t he col-19 promoter to the green fluorescent protein gene (gfp) and dem onstrated that hypodermal expression of the fusion gene is adult-speci fic in wild-type animals and temporally regulated by the heterochronic gene pathway. We generated a transgenic strain in which the col-19::g fp fusion construct is not expressed because of mutation of lin-4 whic h prevents seam cell terminal differentiation. We have identified and characterized 26 mutations that restore col-19::gfp expression in the lin 4 mutant background. Most of the mutations also restore other aspe cts of the seam cell terminal differentiation program that are defecti ve in lin-4 mutant animals. Twelve mutations are alleles of three prev iously identified genes known to be required for proper timing of hypo dermal terminal differentiation. Among these are four new alleles of l in-42, a heterochronic gene for which a single allele had been describ ed previously. Two mutations define a new gene, lin-58. When separated from lin-4, the lin-58 mutations cause precocious seam cell terminal differentiation and thus define a new member of the heterochronic gene pathway.