Worm Breeder's Gazette 14(1): 28 (October 1, 1995)
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.
Biology Department, Indiana University, Bloominton, IN 47405
We have been studying maternal-effect sterile (mes) mutants in an
effort to understand how development of the germ line of C. elegans is
maternally controlled. Four genes, mes-2, mes-3, mes-4, and mes-6, all
give a similar mutant phenotype. Specifically, progeny of mes/mes mothers
undergo apparently normal embryogenesis and develop into heathy appearing
adults but fail to produce a functional germ line. Mes-2 mutant larvae
can produce as many as 80 germ nuclei by the L4 larval stage. There is
then an obvious and extensive die-off of germ nuclei, which results in
adult worms having only ~ nine germ nuclei and no mature gametes (Capowski
et al., 1991; Paulsen et al., 1995).
mes-3 and mes-6 have previously been cloned (mes-3: Paulsen et
al., 1995); they encode proteins that lack significant similarity to any
described proteins. Transformation rescue of Mes-2 mutants was
accomplished by injecting overlapping genomic fragments of 7 and 13 kb in
length from cosmid R06A4 on LGIIR. These fragments, which both recognize
a 2.5 kb Northern transcript, were used to screen a cDNA library. A
candidate cDNA clone of 2.5 kb, which also recognizes the 2.5 kb
transcript, was identified in the screen. Injection of both sense and
antisense RNA produced from this cDNA phenocopied the Mes-2 mutant
phenotype in wild-type worms. Worms injected with control RNA did not
show this phenotype. The mes-2 cDNA encodes a protein that contains a
150 amino acid motif known as the SET domain. This motif gets its name
from three Drosophila proteins in which it is found: Trithorax (Trx),
Suppressor of variegation 3-9 [Su(var)3-9], and Enhancer of zeste [E(z)]
(see WBG for figure). SET domains have also been found in proteins from
plants, yeast, and mammals.
The E(z) protein, to which MES-2 is most similar, is a member of
the Polycomb group (PC-G) genes in Drosophila. These genes are believed
to be needed for the long-term maintenance of transcriptional inactivation
of specific genes, such as the homeotic genes. PC-G proteins are thought
to work by affecting chromatin structure, although they do not seem to
bind DNA directly. Current thinking is that these proteins form large
complexes with other gene products and control chromatin structure at
specific sites (reviewed in Paro, 1995).
Genetic evidence suggests that the Mes mutant phenotype may be
due to defects in X-chromosome gene expression, specifically in the germ
line. The similarity of MES-2 to E(z) and other SET domain-containing
proteins, which have a long-term role in regulation of expression, is
intriguing in light of our suspicion that mes genes may affect gene
expression on the X chromosome. We are in the process of raising
antibodies to MES-2 in order to further analyze this possibility.
References:
E. Capowski, P. Martin, C. Garvin, and S. Strome (1991) Genetics 129,
1061-1072.
J. Paulsen, E. Capowski, and S. Strome (1995) Genetics, in press.
R. Paro, TIG (1995) 11 (8), 295-297.