Trypanosomatid transcription factors: waiting for Godot.

K inetoplastid protozoa include the parasites Trypanosoma brucei, Trypanosoma cruzi, and various species of Leishmania that are responsible for African trypanosomiasis, Chagas’ disease, and leishmaniasis, devastating diseases that aff lict millions of victims largely in the tropical regions of the globe. In addition to eliciting long-standing interest from medical microbiologists, these intriguing protists have become more recently the objects of intense scrutiny by cellular and molecular biologists. Because these singlecell eukaryotes occupy a very ancient position on the evolutionary tree (1), they exhibit many unusual biological characteristics that distinguish them from their mammalian hosts and from many other organisms used as model systems by biologists. The increasing number of molecular, genetic, and cellular studies on these unusual protozoa have thus allowed a window into various biological phenomena that have often been obscured in the more routinely studied organisms. In some cases, the apparent peculiarities of these protists have served simply to highlight phenomena that are widespread within the biological universe. Accordingly, glycosylphosphatidyl inositol (GPI) membrane anchors were first discovered in T. brucei (2, 3); their discovery was facilitated by the abundance of the variant surface glycoproteins that cover the surface of this parasite, and which are involved in antigenic variation and tethered into the extracellular face of the plasma membrane by GPI moieties. Subsequent studies revealed that many eukaryotes, including mammalian cells, also express a plethora of GPI-linked membrane proteins. In other cases, the idiosyncrasies have proven to be more esoteric, one example being the astonishing RNA editing that extensively alters the sequence of many mitochondrial transcripts (4) and which has correlates among other protozoa, though not, apparently, among higher eukaryotes. An article by Das and Bellofatto (11) in this issue of PNAS now advances our rather rudimentary knowledge of transcription by RNA polymerase II (Pol II) in the Kinetoplastida. One remarkable phenomenon in gene expression among the Kinetoplastida is the existence of a 39-nt spliced leader (SL) sequence that is placed onto the 5 ends of all mRNAs by a trans-splicing reaction (5). Although such trans-splicing has been observed subsequently for a subpopulation of mRNAs in other eukaryotes such as Caenorhabditis elegans, the distinctive feature of the kinetoplastid protozoa is the universal requirement for SL addition onto every mRNA yet analyzed. The SL sequence is derived from a larger transcript (ranging in size from 96to 141-nt, depending upon the species of parasite) that is transcribed from arrays of tandemly repeated genes. This SL RNA is thus an abundant small nuclear RNA (snRNA). Transcription of SL RNA, which initiates from a separate promoter upstream of each gene, had been ascribed previously to RNA Pol II, RNA Pol III, or an RNA polymerase of intermediate character (6). More recent studies by the Bellofatto group (7) have confirmed