The genome of african trypanosome

African trypanosomes of the species Trypanosoma brucei are extracellular protozoan parasites of the mammalian bloodstream. They survive for long periods in the host bloodstream though a process of antigenic variation involving periodic switching of the major cell surface protein, variable surface glycoprotein. To facilitate this important survival mechanism, there exists in T. brucei a highly specialised gene organisation that includes a large number of minichromosomes (MCs). The presence of such MCs is intimately linked to VSG switching – only Kinetoplastidae that undergo antigenic variation possess MCs.

The nuclear genome of T. brucei can be divided into 3 chromosome classes. The megabase-sized chromosomes (MBCs) are 0.9–6 Mb in size. They are diploid and form 11 homologous pairs which show considerable variation in size both between homologues and between strains. All of the housekeeping genes of the organism are contained on MBCs, and VSG expression sites (VSG-ESs) are found at most, if not all, telomeres. The intermediate chromosomes (ICs) are 200–700 kb in size. They number between 1–7 in most strains and are of uncertain ploidy. Little is known of the function of this class of chromosomes except that they share repetitive elements with MCs but, like MBCs, contain VSG-ESs. The minichromosomes (MCs) are small (30–150 kb), linear and very numerous. A population of ~100 MCs, comprising ~10% of the nuclear DNA, is maintained by T. brucei as a means of expanding the number of available telomeric VSG genes (VSGs). Loci from this repertoire of MC VSGs, despite being fewer in number than MBC-internal VSGs, are preferred genes for antigenic switching events early in parasitaemia.

We have found the central 177 bp repeat core to make up ~60% of total MC DNA and that the central core is a large repetitive palindrome with an point of inversion around the centre of the MC.