In an effort to identify genes that act early in embryonic development, we isolated two allelic ems-induced mutations (w1 and e2482; formerly called zen-1) that result in defects in embryogenesis beginning at the 28-cell stage ('91 and '93 Worm Meeting abstracts). To characterize its requirement in early embryonic development, we isolated molecular clones of this locus. A cosmid (K07H12) was identified that rescues the embryonic lethality of both alleles and a 1 kb fragment from this cosmid was found to contain the rescuing activity. The sequence of this fragment is virtually identical to the published sequence of the 1 kb repeat from the 110 kb rrs-1_cluster. The repeat encodes both 5S rRNA and SL1 RNA. Southern analysis demonstrated that e2482 and w1 completely delete the rrs-1 cluster. We found that the SL1 gene alone is sufficient to rescue the embryonic lethality of e2482 and w1, while transformation of the 5S rRNA gene alone has no effect on the phenotype. Rescue was abolished when SL1 constructs containing either of two mutations in the conserved Sm binding site were used. Thus, SL1 RNA performs an essential function in embryogenesis and its ability to be trans-spliced appears to be required for this function. Initial experiments indicate that transformation of the SL2 gene can also rescue these mutants, suggesting that the essential role performed by the SL1 leader is also contained within SL2. We are further examining the requirement for SL1 in early embryogenesis by investigating the role played by SL1 in proper metabolism of embryonic trans-spliced messages.
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