Trans-Splicing Enhances Processivity of RNA Polymerase II and Suppresses Polyadenylation Signals

Trans-splicing, the process where exons from two different pre-mRNA molecules are joined, is an efficient mechanism for generating diverse isoforms encoded by a single locus. This process has been most thoroughly studied in the mod(mdg4) locus of Drosophila. This locus comprises one gene encoding the constant N-terminal part of the Mod(mdg4) protein and five genes that encode over thirty 3'-exons clustered together, determining the diversity of C-terminal domains. In this study, transgenic lines were obtained where a strong polyadenylation signal from the SV40 virus was inserted into two 3'-exon clusters with the aim of halting transcription and, consequently, reducing the frequency of trans-splicing between the constant exons of the mod(mdg4) gene and the 3'-exons of the modified clusters. Unexpectedly, it was found that SV40 polyadenylation signals had no effect on either the transcription level of the clusters or the efficiency of trans-splicing. The results indicate the ability of RNA polymerase II, transcribing the 3'-exons of the mod(mdg4) locus, to overcome even strong polyadenylation signals from SV40.