Elements regulating cell- and stage-specific expression of the C. elegans MyoD family homolog hlh-1.

We investigated the cis-acting sequences regulating expression of the Caenorhabditis elegans gene hlh-1, a homolog of the MyoD family of myogenic regulatory factors. The hlh-1 gene is expressed in mature body wall muscle, in clonal muscle precursors, in a set of early embryonic blastomeres (the MS-granddaughters), and in six glial-like cells called GLRs. The entire natural hlh-1 expression pattern is recapitulated in transgenic animals containing an hlh-1::lacZ fusion with 5.1 kb of hlh-1 sequences beginning upstream of the coding region and extending into the second exon. Deletions and rearrangements in this 5.1-kb sequence were assayed for their effects on reporter gene expression in transgenic animals. Deletions removing a segment 434-550 bp upstream of the coding region resulted in a loss of all aspects of the expression pattern, suggesting that at least one common regulatory factor is required for all expression. Deletions of other regulatory elements affected distinct aspects of the expression pattern; hence, each expression subpattern exhibits a different set of sequence requirements. Interspecies sequence comparison and lacZ expression constructs with the related nematode Caenorhabditis briggsae indicated that the C. briggsae and C. elegans genes contain equivalent sets of control signals. The results from this work implicate the hlh-1 promoter as an integration point for diverse temporal and spatial control signals.