Homeobox genes are highly conserved transcription factors that play key roles in the development of humans and animals. From a 4D microscopy expression screen performed in the Burglin lab, a homeobox gene -
ceh-5 - displaying a unique asymmetrical expression was revealed. This ortholog of the human VAX genes is expressed in three different groups of cells during gastrulation. An interesting aspect of
ceh-5 expression in the two bilaterally symmetric cell groups is a left-right asymmetry in their expression levels. Little is understood about cell fate specification after the beginning of gastrulation, and also how left-right asymmetries are generated;
ceh-5, therefore, provides an excellent entry point to study these phenomena.
We are dissecting the promoter region of
ceh-5 to understand how the distinct spatio-temporal expression is generated. We created a series of deletions in the promoter region of
ceh-5 fused to GFP that were used to make transgenic animals. The GFP expression patterns in the transgenic worms were monitored using 4D-imaging. 790bp upstream of the ATG is sufficient to initiate and maintain
ceh-5's expression during embryogenesis. Within this region, one of the motifs crucial to drive
ceh-5 complex expression is a SKN-1 binding motif.
Another key motif revealed by the investigation is a 21bp in the promoter region containing an E-box motif essential for the regulation of
ceh-5. This motif is highly conserved among different Caenorhabditis species and is known to be a binding site for basic helix-loop-helix (bHLH) transcription factors (1). In order to look for transcription factors involved in regulating the expression of
ceh-5, we screen via RNAi knockdown, searching for candidates that would create an altered
ceh-5 expression pattern. Although the screen is not yet saturated, we have identified -
hlh-2,
ngn-1 and
cnd-1 - when knocked-down, abolishes a part of the
ceh-5 expression pattern.
1) Yamada, K. and Miyamoto, K. 2005. Front Biosci.