C. eleganssequences that controltrans-splicing and operon pre-mRNA processing

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Abstract
Many mRNAs inCaenorhabditis elegansare generated through atrans-splicing reaction that adds one of two classes of spliced leader RNA to an independently transcribed pre-mRNA. SL1 leaders are spliced mostly to pre-mRNAs from genes with outrons, intron-like sequences at the 5′-ends of the pre-mRNAs. In contrast, SL2 leaders are nearly exclusivelytrans-spliced to genes that occur downstream in polycistronic pre-mRNAs produced from operons. Operon pre-mRNA processing requires separation into individual transcripts, which is accomplished by 3′-processing of upstream genes and spliced leadertrans-splicing to the downstream genes. We used a novel computational analysis, based on nonnegative matrix factorization, to identify and characterize significant differences in thecis-acting sequence elements that differentiate various types of functional site, including internal versus terminal 3′-processing sites, and SL1 versus SL2trans-splicing sites. We describe several key elements, including the U-rich (Ur) element that couples 3′-processing with SL2trans-splicing, and a novel outron (Ou) element that occurs upstream of SL1trans-splicing sites. Finally, we present models of the distinct classes oftrans-splicing reaction, including SL1trans-splicing at the outron, SL2trans-splicing in standard operons, competitive SL1-SL2trans-splicing in operons with large intergenic separation, and SL1trans-splicing in SL1-type operons, which have no intergenic separation.