ACTIVATION OF THE MEC-3 PROMOTER IN 2 CLASSES OF STEREOTYPED LINEAGESIN CAENORHABDITIS-ELEGANS

The mec-3 gene of Caenorhabditis elegans encodes a homeodomain protein and is expressed in one of two cells upon asymmetric cell division. A s a result of asymmetric mec-3 expression, the two sister cells expres s different fates, so mec-3 is a likely target for the machinery that mediates asymmetric cell division. The unc-86 gene encodes a homeodoma in protein of the POU family, which activates mec-3 by binding to its promoter. The ten mec3-expressing cells are a subset of the anterior d aughters of UNC-86-containing cells. Posterior daughters of UNC-86-con taining 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, UN C-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 determin e 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 t erminal PVDR neuron, one cell division after unc-86 is expressed. mec- 3 expression in PVDR can occur by transcriptional regulation alone; se gregation of the mec-3 RNA or protein is not required to explain the a symmetric expression of mec-3. In the Type: II Q lineage, the mec-3 pr omoter 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 likel y that, in the short-lived immediate anterior daughter cell in Type II lineages, mec-3 product does not accumulate to levels that can influe nce subsequent events. Our results suggest that the mec-3 promoter is activated in all anterior daughters of unc-86-expressing cells.