Worm Breeder's Gazette 13(5): 66 (February 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.
Department of Biochemistry, University of Wisconsin-Madison, Madison, WI
53706
In screens for zygotic genes that act early in embryogenesis, we
isolated two EMS-induced alleles (e2482 and w1) of a locus which we
called zen-1 (1). Both alleles cause cell migration defects beginning
at the onset of gastrulation, a failure to undergo compaction and
morphogenesis, and cell fate specification defects, suggesting an early
zygotic requirement for this locus.
By deficiency and polymorphism mapping we narrowed the region of
this locus to ~400 kb within the unc-76 - dpy-21 interval. Pools of
cosmids in this region were scored for their ability to rescue embryonic
lethality. One cosmid, K07H12, was identified that rescued the
lethality. We found that K07H12 consists mostly of 1 kb repeats from the
rrs-1 cluster. rrs-1 consists of 110 tandemly repeated 1 kb units, each
of which encodes both 5S rRNA and the trans-spliced leader RNA, SL1.
Surprisingly, a 1 kb repeat alone rescues the embryonic lethality of both
of our alleles, suggesting that both mutations might be deletions of the
rrs-1 cluster. Indeed, Southern analysis of homozygous embryos confirmed
that the cluster is completely deleted in both mutants.
To determine whether the embryonic lethality of these deletions
results from the lack of SL1, 5S rRNA, or both, the genes encoding SL1
and 5S rRNA were tested individually. We found that SL1 is both
necessary and sufficient for rescue of embryonic lethality; however,
hatching animals generally arrest at early larval stages, presumably
owing to the lack of zygotic 5S rRNA. A 3 bp deletion in the 8 bp Sm
binding site of the SL1 outron [which is required for splicing in vitro
(2)] abolishes rescue, suggesting that the ability of SL1 to be trans-
spliced is essential for its rescuing activity.
What is the fate of embryos deficient for SL1? Mutations that
remove SL1 in the rrs-1 cluster affect many processes during
embryogenesis. Thus, SL1 may be essential for the activity of many RNAs.
This is not unexpected since SL1 is trans-spliced to ~56% of mRNAs in C.
elegans (3). However not all zygotic transcripts appear to require SL1,
as a number of cell types are properly specified in these mutants (1).
Perhaps some mRNAs which are normally trans-spliced to SL1 do not need
SL1 to function properly, as has been demonstrated for rol-6 (4). These
findings demonstrate that a trans-spliced leader, and probably the
process of trans-splicing, are essential for early embryonic development.
Using our mutants we plan to address the normal role of SL1 in metabolism
and/or expression of trans-spliced messages. (For example, is SL1
required for normal processing, transport, stability, or translation of
trans-spliced RNAs?)
1) WBG 11: 4, p. 92; Worm meeting abstracts: p. 145 (1991); p. 127
(1993).
2) Maroney, P.A., et. al. EMBO Journal 9, 3667-3673, (1990).
3) Spieth, J. et. al. Cell 73, 521-532 (1993).
4) Conrad, R. et. al. EMBO Journal 12, 1249-1255, (1993).