Rapid trans Splicing Precedes Polyadenylation of Newly Synthesized Tubulin Transcripts ELISABETITA ULLU,"12 KEITH R. MATTHEWS,'t AND CHRISTIAN TSCHUDI'*

Many trypanosome genes are expressed as part of large polycistronic transcription units. This finding suggests that regulation ofmRNA biogenesis may emphasize RNA-processing reactions more so than in other organisms. This study was undertaken to understand the temporal order of two RNA-processing reactions, trans splicing and polyadenylation, in the maturation of trypanosome mRNAs in vivo. Kinetic studies revealed rapid trans splicing of ce-tubulin, 13-tubulin, and actin pre-mRNAs within 1 to 2 min after synthesis of the 3' splice site. Furthermore, following blockage of pre-mRNA synthesis, newly synthesized spliced leader RNA cannot be used for trans splicing, suggesting that trypanosomes do not accumulate substantial amounts of pre-mRNA which can provide splice acceptor sites. Thus, trans splicing is cotranscriptional. In addition, we show that trans splicing precedes polyadenylation in the processing of trypanosome tubulin pre-mRNAs. The biosynthesis ofmRNA in Trypanosoma brucei differs in at least two aspects from that in other eukaryotic organ-isms. First, it appears that protein-coding genes are orga-nized in polycistronic transcription units giving rise to RNA primary transcripts that contain more than one mRNA coding region in the same topological arrangement as in th