Null mutants of the C. elegans labial/Hox1 type gene
ceh-13 display severe morphological defects, predominantly in the anterior, and cell adhesion defects 1 . Since spaciotemporal
ceh-13 expression appears to be controlled at the transcriptional level we have undertaken a promoter analysis. Interestingly, the
ceh-13 promoter region is capable of driving reporter gene expression in Drosophila melanogaster in a pattern highly reminiscent of labial (lab) expression. As for endogenous lab, this expression depends on the presence of lab function and on wingless (wg) signaling. In the worm, important features of comma stage expression also depend on a highly conserved auto-regulatory input that requires the activities of
ceh-13 and
ceh-20, the worm extradenticle homolog. Consistent with that, a 450bp long enhancer fragment (
enh450) that drives GFP expression in most cells that express
ceh-13 at this stage contains a 10 bp element identical or very similar to the so called auto-regulatory elements of mouse hoxb1 and Drosophila lab respectively. Point mutations in this element abolish the activity of
enh450 (see also abstract by Takacs-Vellai et al.). A different regulatory element of 740bp (
enh740) is sufficient to drive correct early embryonic
ceh-13 expression. Interestingly, this fragment acts as a strong wg responsive element in fly embryos. In vitro this element binds POP-1, the downstream effector of the Wnt/WG signaling pathway in C. elegans , indicating that it might also act as a target for this type of signaling in the worm. This is supported by our finding that in
pop-1 (RNAi) and in
lit-1 mutant early embryos CEH-13::GFP is absent from most of the cells that normally express it. However, POP-1 may not act simply as an activator of
ceh-13 . In the AB lineage e.g. onset of
ceh-13 expression occurs in the posterior daughters of a/p dividing cells that have lower POP-1 levels than their anterior sisters that don’t express
ceh-13 . This indicates that POP-1 might act to activate or repress
ceh-13 depending on its concentration or phosphorylation status. Furthermore, sub-fragments of
enh740 that interact efficiently with POP-1 in vitro but lack the most upstream portion are not capable of activating reporter gene expression suggesting that additional spaciotemporal cues are required for correct activation of
ceh-13 . We are currently further investigating the regulatory elements within the
ceh-13 promoter using a combination of deletional analysis, in vitro binding assays and comparative sequence analysis of
ceh-13 from C. elegans , C. briggsae and C. remanei . 1) Brunschwig et al. (1999) Development 126:1537-1546