The coordinated specification and deployment of cells in space and time is central to animal development. The Drosophila hunchback (hb) gene codes for a transcription factor that is well known for its role in the spatial patterning of the embryonic nervous system, and recently this factor has also been implicated in the temporal regulation of neuronal birth. This means that the activity of hb needs to be confined to a discrete time window. How is this achieved? According to two new reports in Developmental Cell, the hunchback homologue hbl-1 in the worm Caenhorhabditis elegans might provide some vital clues.

Abrahante et al. and Lin et al. showed that loss of hbl-1 function in the C. elegans larva causes certain cells, including the seam cells of the hypodermis, to acquire adult characteristics prematurely. This indicates that HBL-1 is involved in regulating the timing of the larva–adult transition. The teams also showed that HBL-1 protein expression is normally downregulated in the hypodermis and ventral nerve cord (VNC) during post-embryonic development.

The downregulation of HBL-1 depends on the presence of the 3' untranslated region (3'UTR) of the hbl-1 gene. This region contains binding sites for microRNAs (miRNAs) — small non-coding RNAs that regulate gene expression at the post-transcriptional level. Abrahante et al. showed that a miRNA called let-7 binds to the 3'UTR, and is required for the downregulation of HBL-1 in the VNC. Lin et al. provided additional support for this idea, and showed that another miRNA, lin-4, might also be required.

Could the expression of the hb gene product in Drosophila be regulated by miRNAs? Such a mechanism would certainly account for the rapid switching of transcription factor expression in neuroblasts, which controls the temporal sequence of neuronal specification in the Drosophila embryo. Also, the 3'UTR of the hb gene has been found to contain putative miRNA binding sites, so this possibility will definitely be worth exploring.