VSG gene control and infectivity strategy of metacyclic stage Trypanosoma brucei

As the metacyclic trypanosome stage develops in the tsetse fly salivary glands, it initiates expression of variant surface glycoproteins (VSGs) and does so by each cell activating, at random, one from a small subset of metacyclic VSG (M-VSG) genes. Whereas differential activation of individual VSG genes in the bloodstream occurs as a function of time, to evade waves of antibody, it is believed that the aim in the metacyclic stage is simultaneously to generate population diversity. M-VSG genes are activated in their telomeric loci and belong to monocistronic transcription units, unlike all other known trypanosome protein-coding genes, which appear to be transcribed polycistronically. The promoters of these metacyclic expression sites (M-ESs) have the unique property, in this organism, of being switched on and off in a life-cycle stage specific pattern. We have found that the 1.22 M-ES promoter is regulated according to life cycle stage, differential control being exerted through different elements of the promoter and under the influence of its genomic locus. We have characterized in detail the telomeres containing the 1.22 and 1.61 M-ESs. Upstream of the M-ES is a possibly haploid, non-transcribed region with some degenerate sequences homologous with expression site associated genes (ESAGs) that occur in bloodstream VSG expression sites. Further upstream (respectively, 22 and 13 kb upstream of the 1.22 and 1.61 VSG genes) are alpha-amanitin sensitive transcription units that may be polycistrons and are transcribed in all examined life cycle stages. They contain a number of genes. The differences between metacyclic and bloodstream ESs may have important consequences for life cycle regulation, genetic stability, phenotype complexity and adaptability of the metacyclic stage as it infects different host species.