RNA Polymerase Transcription Machinery in Trypanosomes

Transcription is a fundamental biological process employed by all living organisms to decode their genetic information. The information stored in genomic DNA is copied into RNA mol- ecules by polymerization of ribonucleotide building blocks, which ultimately gives rise to different classes of transcripts. mRNAs encode polypeptides, rRNAs drive the macromolec- ular protein-synthesis machinery, and tRNAs act as adaptor molecules to assemble amino acids into proteins. Synthesis of specific transcripts is influenced by environmental and internal cell signals, which in turn are pivotal for the control of cellular regulatory networks. Trypanosomes are unicellular parasitic protozoa, members of the order Kinetoplastidae, which diverged early during evo- lution. They cause a wide range of debilitating diseases in humans and domestic animals. Trypanosoma brucei, known as the African trypanosome, is transmitted by tsetse flies in sub- Saharan Africa (15). Infection fulminates into African sleeping sickness in humans and nagana in animals (3). T. brucei is a digenetic parasite that cycles as a procyclic form in the diges- tive tract of the tsetse vector and as an extracellular blood- stream form in its mammalian host. During its complex life cycle, the parasite passes through five successive morphologi- cally distinct forms (39). Parasites change from the procyclic form, which is characterized by a procyclic-specific surface coat, through two morphologically distinct forms in the fly and then they emerge as long, slender bloodstream forms, covered with a variant surface glycoprotein coat. Once inside the mam- malian host, the long slender bloodstream form actively divides and establishes parasitemia. In the late phases of infection, the morphology of the parasite changes to nondividing short stumpy forms, which are ready to be taken up by the insect during a blood meal. The bloodstream form, with a rudimen- tary mitochondrion, is perfectly adapted to utilize the abun- dant supply of glucose from the mammalian blood and gener- ate sufficient energy by glycolysis. The insect form, on the other hand, has a functional mitochondrion and generates most of its energy by respiration. These necessary metabolic adaptations depend upon a precise orchestration of numerous metabolic and cell biological activities. Studies of trypanosomes have uncovered several unusual biological phenomena (6). Notable among them are trans splic- ing and RNA editing (reviewed in references 7, 35, 46, 48, and 59). Protein-coding genes in trypanosomes are transcribed as long polycistronic precursor RNAs. Individual mature mRNAs are formed by trans splicing of a 39-nucleotide spliced leader (SL) RNA at the 5 end and subsequent 3 end maturation. RNA editing, used to produce mitochondrial mRNA, requires extensive alterations of primary transcripts by guide RNAs. Although guide RNA-dependent RNA editing is uniquely ob- served in trypanosomes, trans splicing has subsequently been observed in several other lower eukaryotes, including nema- todes, trematodes, euglenoids, and chordates. Therefore, stud- ies of trypanosomes are expected to uncover cryptic mechanis- tic components of eukaryotic biology and reveal exotic cellular processes.