Transcription in trypanosomes: a different means to the end

The recent publication of T. brucei genome has enabled a new wave of molecular and genetic experiments, which have resulted in a clearer picture of the transcription machinery in this organism. While trypanosomes possess most of the subunits common to the three RNA polymerases typically found in eukaryotes, a number of interesting differences exist, presumably reflecting their early divergence from the eukaryote crown lineage. Most notably, the absence of many class-specific RNAP subunits, and paucity of general transcription factors, suggests that the intricate, and varied, mechanisms for regulation of transcript initiation in higher eukaryotes arose after this divergence. It appears that the Tritryp transcriptional apparatus may be the most primitive yet identified in the Eukarya, containing perhaps a single shared set of GTFs for transcription initiation. Indeed, the RNAP I and II promoters described in T. brucei share structural similarities with the RNAP III type III promoters of higher eukaryotes, and some protein-coding genes are transcribed by RNAP I. The bidirectional RNAP II-mediated transcription initiation regions separating the long polycistronic clusters of protein-coding genes in trypanosomatids may represent ancestral, less sequence-specific, promoters mostly replaced in other eukaryotes by the archetypal TATA-containing promoters. Unlike most other organisms where transcriptional mechanisms plays a major role in regulation of mRNA levels, control of mRNA abundance in trypanosomes occurs mainly at the steps of RNA processing, stability and degradation. This reliance on post-transcriptional regulation of gene expression may explain the relative dearth of transcription factors and over-representation of RNA-binding proteins in the trypanosome genome. Introduction In all living organisms the genetic information encoded within the DNA genome must be transcribed into the RNA molecules that either act directly, or are translated into proteins, to carry out the myriad processes involved in cellular maintenance and growth. RNA polymerases are multisubunit protein complexes that are recruited to specific sites (promoters) within the genome by general transcription factors that are specific for each polymerase. In most eukaryotes, this process is differentially regulated according to developmental or proliferation state by modification of chromatin, as well as recruitment of specific transcription factors and mediator complexes. This leads to initiation of the tran-