| During the embryonic development of multicellular organisms, different cell types arise. Since all cells carry an identical genome, the difference in cell differentiation is primarily brought about by differential gene activation and repression. The two key elements that coordinate this regulatory mechanism are the cis-acting elements located on the genes to be regulated and the trans-acting proteins that recognize these cis-elements. Taking together with the results of characterization of ram-1 and ram-4, cloning of ram-2 in this study revealed that there are three collagen genes involved in male sensory ray morphogenesis of Caenorhabditis elegans. These ram genes are required to determine the proper morphology of sensory rays in the male tail. Using nlscfp reporter gene driven by the corresponding promoter of each ram gene, we showed that the expression of these ram genes was switched on only in the male hypodermis from late L4 to young adult stage, and they shared a largely overlapping expression pattern. To delineate the temporal and spatial specificity, we identified the critical cis-acting elements of ram-1, ram-2 and ram-4 promoters by deletion analysis. Two cis-acting elements were defined on each of these promoters, one controlling the level of transcription as revealed by reporter signal, and the other is essential for the biological activity shown in mutant rescue assays. These two elements are in proximity in the ram-2 promoter, and presumably function as one complex element. They were used in a yeast one-hybrid screen to identify potential transcription factor(s) that regulate ram-2 expression in C. elegans. Three candidate genes were identified in this screen. RNAi knock down of these genes in reporter transgenic animals confirmed their impact on ram-2 transcription, although they did not appear to coordinate all three ram collagen genes. Based on the results of the examination of their expression profiles, the phenotypic defect upon expression attenuation and the availability of nucleic acid binding domain, we suggest that spt-5 is a potential transcription factor regulating ram-2 transcription. We envisage that the identification of these cis-elements and the regulator molecule will provide an entry for us to further investigate the transcriptional regulation of tail specific collagens in C. elegans and thus shed light on developmental coordination of gene activities. |
