The
mec-3 gene of Caenorhabditis elegans encodes a homeodomain protein and is expressed in one of two cells upon asymmetric cell division. As a result of asymmetric
mec-3 expression, the two sister cells express different fates, so
mec-3 is a likely target for the machinery that mediates asymmetric cell division. The
unc-86 gene encodes a homeodomain protein of the POU family, which activates
mec-3 by binding to its promoter. The ten
mec-3-expressing cells are a subset of the anterior daughters of UNC-86-containing cells. Posterior daughters of UNC-86-containing cells do not express
mec-3, even though the UNC-86 protein is distributed into both daughter cells. Lineages that express the
unc-86 and
mec-3 genes can be grouped into two types: in Type I lineages, UNC-86 protein is first made in the immediate parent of the terminal
mec-3-expressing cell, while in Type II lineages, UNC-86 is first made in the grandparent of the terminal
mec-3-expressing cell. The purpose of experiments presented here is to understand the relationship between the
mec-3 expression patterns in each type of lineage, and to determine the fundamental activity pattern of the
mec-3 promoter. We find that in the Type I V5.pa lineage,
mec-3-lacZ is first synthesized in the terminal PVDR neuron, one cell division after
unc-86 is expressed.
mec-3 expression in PVDR can occur by transcriptional regulation alone; segregation of the
mec-3 RNA or protein is not required to explain the asymmetric expression of
mec-3. In the Type II Q lineage, the
mec-3 promoter activity can be detected in the immediate anterior daughter of the first
unc-86-expressing cell, but when this cell divides,
mec-3 is expressed in only one of its daughters at later times. It seems likely that, in the short-lived immediate anterior daughter cell in Type II lineages,
mec-3 product does not accumulate to levels that can influence subsequent events. Our results suggest that the
mec-3 promoter is activated in all anterior daughters of
unc-86-expressing cells.