C. elegans sym-1 is a downstream target of the Hunchback-like-1 developmental timing transcription factor

In the nematode Caenorhabditis elegans, the let-7 microRNA (miRNA) and its family members control the timing of key developmental events in part by directly regulating expression of hunchback-like-1 (hbl-1). C. elegans hbl-1 mutants display multiple developmental timing deficiencies, including cell cycle defects during larval development. While hbl-1 is predicted to encode a transcriptional regulator, downstream targets of HBL-1 have not been fully elucidated. Here we report using microarray analysis to uncover genes downstream of HBL-1. We established a transgenic strain that over-expresses hbl-1 under the control of the heat shock promoter. Heat shock-induced hbl-1 over-expression led to retarded hypodermal structures at the adult stage, opposite to the effect seen in loss of function (lf) hbl-1 mutants. The microarray screen identified numerous potential genes that are up-regulated or down-regulated by HBL-1, including sym-1, which encodes a leucine-rich repeat protein with a signal sequence. We found an increase in sym-1 transcription in the heat shock-induced hbl-1 over-expression strain, while loss of hbl-1 function caused a decrease in sym-1 expression levels. Furthermore, we found that sym-1(lf) modified the hypodermal abnormalities in hbl-1 mutants. Given that SYM-1 is a protein secreted from hypodermal cells to the surrounding cuticle, we propose that the adult-specific cuticular structures may be under the temporal control of HBL-1 through regulation of sym-1 transcription.