Figure 3.

mRNA processing. (a) This shows the region between two different open reading frames, ORF1 and ORF2. The distance between the polypyrimidine tract and polyadenylation site in T. brucei is about 100 nt. The nature of the interaction between the polyadenylation and splicing complexes is unknown; hypothetical linking proteins are shown but the interaction might instead be direct. (b) The simplest possibility for processing the primary transcript in (a), when only a single splicing signal is present. (c) This is a precursor with two possible splicing signals, labelled as A and B. If neither signal is used, the mRNA will contain more than one open reading frame, and only protein (1) will be produced. (d) If both sites from precursor (c) are used, the RNA from ORF1 (i) has a short 3′-UTR, RNA (2) has a short 5′-UTR (ii) and there will be an additional processed RNA with no open reading frame from intergenic region (iii). This pattern has been documented in detail for the procyclin [101,102] mRNAs but is doubtless seen in many more [29]. (e) If only site A of precursor (c) is used, the RNA for ORF2 will have a long 5′-UTR (iv) that includes splicing signal (b). (f) If only the distal site B f precursor (c) is used, the ORF1 mRNA will have a long 3′-UTR (v) which includes polypyrimidine tract (a).