Dominant gain-of-function mutations that lead to misregulation of the C. elegans heterochronic gene lin-14, and the evolutionary implications of dominant mutations in pattern-formation genes

Abstract

The heterochronic gene lin-14 controls the temporal sequence of developmental events in the C. elegans postembryonic cell lineage. It encodes a nuclear protein that is normally present in most somatic cells of late embryos and LI larvae but not in later larval stages or adults. Two lin-14 gain-of-function mutations cause an inappropriately high level of the lin-14 nuclear protein late in development. These mutations delete 3′ untranslated sequences from the lin-14 mRNAs and identify a negative regulatory element that controls the formation of the lin-14 protein temporal gradient. The 21 kb lin-14 gene contains 13 exons that are differentially spliced to generate two lin-14 protein products with variable N-terminal regions and a constant C-terminal region. No protein sequence similarity to any proteins in various databases was found. The temporal and cellular expression patterns of lin-14 protein accumulation is altered by mutations in the heterochronic genes lin-4 and lin-28. The lin-4 gene is required to down-regulate lin-14 protein levels during the mid-Ll stage. The lin-4 gene product could be the trans-acting factor that binds to the negative regulatory element in the lin-14 3′ untranslated region. In contrast, the lin-28 gene activity positively regulates lin-14 protein levels during early LI. Thus, these genes act antagonistically to regulate the lin-14 temporal switch. The normal down-regulation of lin-14 within 10 h of hatching is not determined by the passage of time per se, but rather is triggered when feeding induces postembryonic development. Loss of lin-28 gene activity causes precocious down-regulation of lin-14 protein levels before feeding, whereas loss of lin-4 gene activity does not affect the level of lin-14 protein before feeding. These data suggest that to trigger the lin-14 temporal switch, the lin-4 gene is up-regulated after feeding which in turn down-regulates lin-14 via its 3′ untranslated region. We speculate on the evolutionary implications of dominant mutations in pattern-formation genes.