Worm Breeder's Gazette 11(4): 102

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What Turns On mec-7? Part II: The Return of mec-3

Michel Hamelin, Jeff Way and Joe Culotti

Figure 1

The mec-7 gene encodes a  -tubulin required for the formation of 15-
protofilament microtubules (Genes and Dev.  3:870).  These large 
diameter microtubules are found exclusively in the touch neurons (ALML 
ALMR, AVM, PVM, PLML, and PLMR) (J.  Cell Biol.93:15).  mec-7 mutants 
lack the 15-protofilament microtubules and are touch insensitive.  We 
recently reported that a mec-7-lacZ extrachromosomal element, evEx1, 
is expressed exclusively in the touch neurons, and at a lesser extent, 
in the FLP neurons (W.B.G.11[3]40).  The gene mec-3 encodes a 
potential transcription factor that is required for the proper 
differentiation of the touch neurons (including the appearance of 15-
protofilament microtubules) (Cell 54:5).  Similarly, mec-3 was 
previously shown to be expressed in the touch neurons and in the FLPs, 
but also in the PVD neurons (Genes & Dev.  3:1823).  An attractive 
possibility is that mec-3 directly activates mec-7 expression.  
However, our mec-7-lacZ construct was found to be expressed at almost 
normal levels in a mec-3 mutant background, using the presumed null 
allele u298:Tc1.  This led us to propose that the mec-3 gene product 
was not required to trigger mec-7 transcription (W.B.G. 11[3]40).  
Here we present recent data which suggest that, in fact, mec-3 is the 
transcriptional activator of mec-7.First, mec-7 may be transcribed in 
all the cells that were previously shown to express a mec-3-lacZ 
fusion gene, i.e.  the touch neurons as well as the FLPs and the PVDs. 
We have stably integrated the mec-7-lacZ fusion gene, following the 
procedure of Kari et al.  (W.B.G.  11[3]14).  The integrated evEx1 
array has been named jeIn1.  Using jeIn1-containing N2 animals, we 
found stronger and more consistent stainings, as compared with evEx1 
containing animals  -Gal activity was still predominantly confined to 
the touch neurons, but a significantly higher proportion of larvae 
stained in the FLPs.  PVD neuron staining was also detected in a 
proportion similar to the FLPs (identities of these cells to be 
confirmed).  mec-7 may therefore be expressed in the exact same subset 
of neurons as mec-3 (this raises the question as to why 15-
protofilament microtubules are not found in the FLPs or the PVDs).
Second, and most importantly, the mec-7-lacZ gene expression is 
greatly reduced in some newly tested mec-3 mutants.  We have 
transferred jeIn1 into the genetic background of 5 more mec-3 alleles (
sent to us by Shohei Mitani).  Table 1 shows the results of the X-Gal 
stainings of these mec-3 transgenics: there is a global decrease, of a 
similar amplitude in all cells, of mec-7 expression.  This decrease is 
more pronounced in some alleles.  This suggests that some alleles of 
mec-3, in particular u298, are somehow leaky, while others, such as u6 
are more severe (but not null?).  In the light of these results, the 
possibility that the mec-3 protein directly activates mec-7 
transcription remains a strong one.
[See Figure 1]

Figure 1