ASYMMETRIC EXPRESSION OF
mec-3 Lili Wang and Jeffrey C.Way Dept. of Biology, Rutgers University, Piscataway, New Jersey 08855-1059
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- expressing sister 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 heat shock 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
w7c-86 (Type I lineages) and those for which the grandparent cell expresses
unc-86 (Type II lineages). jein2 and jeln99 respectively contain about 5 and 600 copies per genome of the 'wild-type'
mec-3-lacZ fusion plasmid pTU28. In jeIn99 but notjeIn2 animals, lacZ expression can be seen in the AIZ, ALN, and SDQ cells, which are the sisters of the
mec-3-expressing cells in Type II lineages (Table lB). (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 o
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-lacZ is present in extremely high copy.