Discontinuous transcription and antigenic variation in trypanosomes

The main theme of this review is the discontinuous synthesis of mRNAs in trypanosomes. This novel process was discovered in the unicellular eukaryote Trypanosoma brucei, but it is probably a general feature of the order of Kinetoplastida, to which several other major human pathogens belong. Discontinuous RNA synthesis involves a sequence of 35 nucleotides (nt) found at the 5' end of all trypanosome mRNAs analyzed. The 35-nt sequence is encoded in arrays of 1.35-kb repeats that are clustered in the genome. The primary transcript of the 1.35-kb repeat is an RNA of 140 nt that carries the 35-nt sequence at its 5' end. The 35-nt sequence is transferred from the 140-nt precursor to pre-mRNAs made elsewhere in the genome. The process has not yet been reconstructed in vitro, and whether transfer involves priming of pre-mRNA synthesis, RNA-RNA ligation followed by splicing, trans-splicing, or more than one of these mechanisms, is still unknown. Circumstantial evidence makes priming the least likely of these alternatives. Why it is advantageous to trypanosomes to make their mRNAs in such an unusual fashion is unclear. As yet, there is no evidence for discontinuous synthesis of mRNAs in organisms other than kinetoplastid flagellates. The mini-exon sequence was first found in mRNAs for Variant-specific Surface Glycoproteins (VSGs), and the control of the synthesis of these proteins is a second theme of this review. Silent VSG genes may be activated by their duplicative transposition to a telomeric expression site. The transposition process looks like a gene conversion, mediated by short blocks of sequence homology. Activation of the transposed gene is due to its insertion into an active transcription unit, i.e. to promoter addition. Telomeric VSG genes can also be activated without duplication. This can occur by a reciprocal translocation in which a silent telomeric gene exchanges position with a gene residing in an active expression site. A VSG gene may also be activated without detectable translocation, however, by the transcriptional activation of the silent expression site in which it is located. How this occurs is still unknown, because the transcription units are so long that the promoter for pre-mRNA synthesis has not yet been reached by chromosome walking. A simple mechanism in which a mobile promoter moves between telomeres has been rendered unlikely by the demonstration that two telomeric transcription units can be simultaneously active when one of them is interrupted by a large DNA insertion.(ABSTRACT TRUNCATED AT 400 WORDS)