Worm Breeder's Gazette 14(5): 65 (February 1, 1997)

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.


Yinhua Zhang, Henrique Ferreira, Scott W. Emmons

Department of Molecular Genetics Albert Einstein College of Medicine 1300 Morris Park Ave. Bronx, NY 10461

        The mab-18 gene product is a putative transcription factor
consisting of the homeodomain plus the carboxy-terminal,
serine-threonine-rich transcriptional activation domain of the C.
elegans Pax-6 homolog (1).  mab-18 is one part of a complex locus that
also includes the gene vab-3.  vab-3 mutations primarily affect
morphogenesis of the head (2).  mab-18 is necessary for expression of
the unique identity of ray 6 in the male tail.  In mab-18 mutants, ray
lineages are unaffected, but cells of ray 6 form a ray at the positon
where ray 4 normally forms, and usually co-assemble with ray 4 cells to
form a large, compound ray.  This phenotype is interpreted as a ray 6 to
ray 4 identity transformation. 
        We have studied the expression pattern of MAB-18 by indirect
immunofluorescence staining with affinity purified anti-MAB-18
antibodies (from a serum raised against a GST-MAB-18 fusion protein). 
We find that MAB-18 first appears in the ray 6 precursor cell R6. 
Staining is localized to the cytoplasm, and is relatively excluded from
the nucleus.  Cytoplasmic staining, and apparent nuclear exclusion,
persist through the two rounds of cell division of the ray sublineage. 
After completion of the sublineage, when the ray cells begin to
differentiate into ray neurons and support cell, staining disappears in
the cytoplasm and appears in the nucleus, where it remains into
        In our previous studies with a mab-18 reporter gene, we
unexpectedly found the reporter was expressed during the ray sublineage
of ray 8, a ray not known to be affected by mab-18 mutations. 
Expression of the reporter in ray 8 cells did not persist into the
adult, but ended after the sublineages were completed.  With the
anti-MAB-18 antibodies, we find cytoplasmic staining in the ray 8
precursor cell, R8, and also in the ray 7 precursor cell, R7.  As for
ray 6, cytoplasmic staining persists through these sublineages. 
However, when the sublineages are completed, staining disappears
altogether, and does not appear in the nucleus.  In several other
non-ray lineages where the antibodies detect MAB-18 or VAB-3, we
observed only nuclear staining.
        We speculate that prior cytoplasmic accumulation of MAB-18 and
sudden release into the nucleus might be important for the rapid
differentiation of ray 6 cells.  Alternatively, or in addition,
regulation of nuclear entry might help direct action of MAB-18 to a
single ray.  One regulatory system might confine transcription of the
mab-18 gene to the three rays 6, 7, and 8, while a second regulatory
system triggers nuclear entry of the MAB-18 protein only in ray 6. 
Recently, Mann and Abu-Shaar (3) have found that nuclear entry of the
homeodomain protein extradenticle in Drosophila embryonic midgut
requires both dpp and wingless signals from visceral mesoderm.  Thus
regulated nuclear entry of homeodomain transcription factors may be a
widespread phenomenon.
1: Zhang, Y., and Emmons, S.W.  (1995)  Nature 377, 55-59.  2: Chisholm,
A.D., and Horvitz, H.R.  (1995)  Nature 377, 52-55.  3: Mann, R.S., and
Abu-Shaar, M.  (1996)  Nature 383, 630-633.