RELATIONSHIP BETWEEN 3'-END FORMATION AND SL2-SPECIFIC TRANSSPLICING IN POLYCISTRONIC CAENORHABDITIS-ELEGANS PRE-MESSENGER-RNA PROCESSING

About 25% of the genes in the nematode Caenorhabditis elegans are in o perons, polycistronic transcription units in which the genes are only 100-400 bp apart. The operon pre-mRNAs are processed into monocistroni c mRNAs by a combination of cleavage and polyadenylation at the 3' end of the upstream mRNA and SL2 trans-splicing at the 5' end of the down stream mRNA. To determine whether 3' end formation and SL2 trans-splic ing are coupled mechanistically, we tested a gpd-2/gpd-3 operon constr uct driven by a C. elegans heat shock promoter, and measured the effec ts of inhibition of 3' end formation and/or trans-splicing on the proc essing of the polycistronic RNA in vivo. The results indicate that pro per 3' end formation of the upstream mRNA in an operon is required for SL2-specificity of downstream mRNA trans-splicing. In contrast, trans -splicing of the downstream mRNA is not necessary for correct 3' end f ormation of the upstream mRNA. In addition, shortening the distance be tween the 5' cap and the AAUAAA of gpd-2 (the upstream gene) decreases the efficiency of 3' end formation and is accompanied by a replacemen t of SL2 with SL1 at the trans-splice site of gpd-3, the downstream ge ne. These results indicate that SL2 frans-splicing, in C. elegans, is coupled mechanistically to 3' end formation in the processing of polyc istronic pre-mRNAs.