In situ analysis of C. elegans vitellogenin fusion gene expression in integrated transgenic strains: effect of promoter mutations on RNA localization.

Expression of the Caenorhabditis elegans vitellogenin (vit) genes is initiated at the larva-to-adult molt in all of the 30 to 34 nuclei of the hermaphrodite intestine. A series of strains in which DNA carrying a vit fusion gene was integrated at low copy number was analyzed by in situ hybridization to determine whether the transgene showed the same tissue-specific expression. Strains with only 247 bp of 5'-flanking DNA accumulated the mRNA product of the introduced vitellogenin gene only in the adult hermaphrodite intestine, and uniformly in all of the intestinal cells. When similar strains carrying vit fusion genes with promoter modifications were tested, no loss of tissue specificity was observed. Surprisingly, however, strains with modified promoters that resulted in reduced levels of expression displayed a novel pattern of transgene RNA localization within their intestines. Strains with severe promoter defects accumulated the transgene mRNA in the central part of the intestine but lacked the mRNA at both ends. Those with less severe promoter mutations lacked the transgene mRNA only in the most anterior intestinal cells. We hypothesize that genes with altered promoters require higher activator concentrations to express the reporter gene, thus revealing an inherent asymmetry in activator levels, lowest in the anterior cells and highest in the central cells of the intestine.