Regulation of rnt-1 expression mediated by the opposing effects of BRO-1 and DBL-1 in the nematode Caenorhabditis elegans.

During development of Caenorhabditis elegans, expression of the RUNX homolog, rnt-1, is tightly regulated both spatially and temporally. In this study, we investigated the mechanism underlying the temporal regulation of rnt-1. We found that rnt-1 contained evolutionarily conserved consensus RUNX binding sequences within one of its introns, and that RNT-1 bound to these intronic sequences both in vitro and in vivo in the presence of BRO-1, suggesting that RNT-1 together with BRO-1 represses its own transcription. Fine deletion and substitution experiments revealed a binding site within the intron that was critical for rnt-1 regulation. Importantly, we found that the TGFbeta homolog, DBL-1, was required for counteracting the repressive activity of BRO-1 at postembryonic stages. Accordingly, ectopic expression of DBL-1 induced transcription of rnt-1 in the lateral hypodermis and other tissues even at the postembryonic stages. Taken together, our data suggest that rnt-1 expression is regulated by the balance between DBL-1-mediated activation and BRO-1-mediated repression at the postembryonic stages.