Of the six blister genes, we have been particularly interested in
bli-6 IV because Bob Herman has shown that alleles of
sqt-1,
dpy-10 and
rol-8 can suppress the dominant mutation
bli-6(
mn4). Although Dpy is often epistatic to Bli, Bob identified non- or very weak Dpy alleles of
dpy-2 and
dpy-10 that suppress
mn4. Since
sqt-1,
dpy-10 and
rol-8 show genetic interactions and all except
rol-8 have been shown to encode collagens (Adam Levy has identified a collagen that is likely to be
rol-8) there is a good possibility that they may physically interact with the
bli-6 gene product. A Bristol/Bergerac polymorphism, identified by hybridizing genomic Southern blots with
col-2 at low stringency, had been previously mapped to the vicinity of
bli-6, and was named
col-4 (Cox et al. Genetics 109:513,1985). Initial three-factor mapping with
unc-5 and
dpy-20 failed to separate
bli-6 and
col-4(Berg), so we decided to examine
col-4 more closely. Further mapping using
unc-44 and
unc-24 showed that
col-4 is approx. 0.4 mu to the right of
bli-6, but concurrent molecular analyses of
col-4 produced some unexpected results. The
col-4 gene was cloned by identifying a
col-2 crosshybridizing clone, in a size selected genomic sublibrary, that showed the expected Br/Bg polymorphism when probed to genomic Southern blots. The first surprise was that
col-4 doesn't encode a collagen. The first case, that we know of, where low stringency collagen cross-hybridization has pulled out something other than a collagen. The
col-4 sequence predicts a polypeptide consisting primarily of 23 tandem copies of the following 15 amino acid repeat: GAPPSGGPPGPF(D/N)PS (note: NPS is a potential glycosylation site)