Heterochromatin-mediated control of virulence gene expression

In recent years, the sequencing and annotation of complete genomes, together with the development of genetic and proteomic techniques to study previously intractable eukaryotic microbes, has revealed interesting new themes in the control of virulence gene expression. Families of variantly expressed genes are found adjacent to telomeres in the genomes of both pathogenic and non-pathogenic organisms. This subtelomeric DNA is normally heterochromatic and higher-order chromatin structure has now come to be recognized as an important factor controlling both the evolution and expression dynamics of these multigene families. In eukaryotic cells, higher-order chromatin structure plays a central role in many DNA processes including the control of chromosome integrity and recombination, DNA partitioning during cell division, and transcriptional control. DNA can be packaged in two distinct forms: euchromatin is relatively accessible to DNA binding proteins and generally contains active genes, while heterochromatin is densely packaged, relatively inaccessible and usually transcriptionally silent. These features of chromatin are epigenetically inherited from cell cycle to cell cycle. This review will focus on the epigenetic mechanisms used to control expression of virulence genes in medically important microbial pathogens. Examples of such control have now been reported in several evolutionarily distant species, revealing what may be a common strategy used to regulate many very different families of genes.