SL1 trans splicing and 3 '-end formation in a novel class of Caenorhabditis elegans operon

Many Caenorhabditis elegans genes exist in operons in which polycistronic p recursors are processed by cleavage at the 3' ends of upstream genes and tr ans splicing 100 to 400 nucleotides away, at the 5' ends of downstream gene s, to generate monocistronic messages, Of the two spliced leaders, SL1 is t rans spliced to the 5' ends of upstream genes, whereas SL2 is reserved for downstream genes in operons. However, there are isolated examples of what a ppears to be a different sort of operon, in which trans splicing is exclusi vely to SL1 and there is no intercistronic region; the polyadenylation sign al is only a few base pairs upstream of the trans-splice site. We have anal yzed the processing of an operon of this type by inserting the central part of mes-6/cks-1 into an SL2-type operon. In this novel context, cks-1 is tr ans spliced only to SL1, and mes-6 3'-end formation occurs normally, demons trating that this unique mode of processing is indeed intrinsic to this kin d of operon, which we herein designate "SL1-type," An exceptionally long po lypyrimidine tract found in the 3' untranslated regions of the three known SL1-type operons is shown to be required for the accumulation of both upstr eam and downstream mRNAs, Mutations of the trans-splice and poly(A) signals indicate that the two processes are independent and in competition, presum ably due to their close proximity, raising the possibility that production of upstream and downstream mRNAs is mutually exclusive.