Worm Breeder's Gazette 13(3): 90 (June 1, 1994)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
mec-3 a homeobox-containing gene that controls the fate of ten mechanosensory neurons in C. elegans, is of special interest because it is expressed upon an asymmetric cell division at the end of a few stereotyped cell lineages in C. elegans. mec-3 is expressed in only one of two daughter cells after a terminal cell division, and because of differential mec-3 expression, the division is asymmetric: in the absence of mec-3 ,both cells can take the fate of the non- mec-3 -expressingsister cell, and when mec-3 is overproduced, both cells can become mechanoreceptors.
The pattern of mec-3 expression is determined by transcriptional enhancers at the mec-3 5' end, rather than by segregation of the mec-3 protein or RNA. To demonstrate enhancer function, mec-3 5' sequences were substituted for one heatshock element in the C. elegans HSP16 promoter, which normally requires two HSEs to function. In the resulting promoter, the remaining HSE functions cooperatively with the mec-3 5' sequences to stimulate transcription in mec-3 cells after heat shock, indicating that the mec-3 5' sequences behave as an enhancer. Specifically, in a mec-3 ( u6 ) ced-3 ( n717 )strain, the PVD cell, but not its undead sister, expresses lacZ upon heat shock.
mec-3 cells can be divided into two groups: those for which the parent cell first expresses unc-86 (Type I lineages) and those for which the grandparent cell expresses unc-86 (Type II lineages). jein2 and jeIn99 respectively contain about 5 and 600 copies per genome of the 'wild-type' mec-3 -lacZfusion plasmid pTU28 .In jeIn99 but not jeIn2 animals, lacZ expression can be seen in the AIZ, ALN, and SDQ cells, which are the sisters of the mec-3 -expressingcells in Type II lineages (Table 1B). (A cell that might be the PVM/SDQ parent is occasionally stained in jeIn99 animals, but this cell is hard to uniquely identify by DAPI staining alone.) We propose that the primary mode of mec-3 expression is to be turned on in anterior daughters of unc-86 cells, but that mec-3 is also repressed in cells that will divide. This repression would be over-ridden in when mec-3 -lacZis present in extremely high copy.