Many Caenorhabditis elegans genes exist in operons in which polycistronic precursors are processed by cleavage at the 3' ends of upstream genes and trans splicing 100 to 400 nucleotides away, at the 5' ends of downstream genes, to generate monocistronic messages. Of the two spliced leaders, SL1 is trans spliced to the 5' ends of upstream genes, whereas SL2 is reserved for downstream genes in operons. However, there are isolated examples of what appears to be a different sort of operon, in which trans splicing is exclusively to SL1 and there is no intercistronic region; the polyadenylation signal is only a few base pairs upstream of the trans-splice site. We have analyzed 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 trans spliced only to SL1, and mes-6 3'-end formation occurs normally, demonstrating that this unique mode of processing is indeed intrinsic to this kind of operon, which we herein designate "SL1-type." An exceptionally long polypyrimidine tract found in the 3' untranslated regions of the three known SL1-type operons is shown to be required for the accumulation of both upstream and downstream mRNAs. Mutations of the trans-splice and poly(A) signals indicate that the two processes are independent and in competition, presumably due to their close proximity, raising the possibility that production of upstream and downstream mRNAs is mutually exclusive.