Alterations in the conserved SL1 trans-spliced leader of Caenorhabditis elegans demonstrate flexibility in length and sequence requirements in vivo

Approximately 70% of mRNAs in Caenorhabditis elegans are trans spliced to c onserved 21- to 23-nucleotide leader RNAs. While the function of SL1, the m ajor C. elegans trans-spliced leader, is unknown, SL1 RNA, which contains t his leader, is essential for embryogenesis. Efforts to characterize In vivo requirements of the SL1 leader sequence have been severely constrained by the essential role of the corresponding DNA sequences in SL1 RNA transcript ion. We devised a heterologous expression system that circumvents this prob lem, making it possible to probe the length and sequence requirements of th e SL1 leader without interfering with its transcription. We report that exp ression of SL1 from a U2 snRNA promoter rescues mutants lacking the SL1-enc oding genes and that the essential embryonic function of SL1 is retained wh en approximately one-third of the leader sequence and/or the length of the leader is significantly altered. In contrast, although all mutant SL1 RNAs were well expressed, more severe alterations eliminate this essential embry onic function. The one nan-rescuing mutant leader tested was never detected on messages, demonstrating that part of the leader sequence is essential f or trans splicing in vivo. Thus, in spite of the high degree of SL1 sequenc e conservation, its length, primary sequence, and composition are not criti cal parameters of its essential embryonic function. However, particular nuc leotides in the leader are essential for the in vivo function of the SL1 RN A, perhaps for its assembly into a functional snRNP or for the trans-splici ng reaction.