Figure 1
From: Parasite epigenetics and immune evasion: lessons from budding yeast
Schematics of varying genes and major mechanisms of variation in Pneumocystis carinii , Trypanosoma brucei , Plasmodium falciparum , and Candida glabrata . (A) In P. carinii, one to three MSG gene arrays (frequently flanked by MSR or PRT genes) are positioned next to a variable number of subtelomeric repeats and the telomeres (depicted by >>>). These genes (black arrows) lack promoters. To be expressed, a single MSG (red arrow) is transferred by homologous recombination to a unique expression site that contains an upstream conserved sequence (UCS). (B) In T. brucei, over 1,000 VSG gene donor sequences (black arrows) are exchanged by homologous recombination to one of fifteen expression sites. The VSG genes at these sites (red arrows) are adjacent to the telomeres and flanked by multiple 70Â bp repeats. Several expression site-associated genes (ESAG s, open arrows) are distal to the telomere. All these and VSG are expressed in the direction of the telomere by one promoter (angled arrow). Only one of the fifteen expression sites is active at a time. Infrequent epigenetic switches of this site confers allelic exclusion and antigenic variation. (C) In P. falciparum, 60 VAR genes are positioned in tandem close to the telomeres or at interchromosomal locations (not shown). VAR A (red arrow) point towards the telomere, while VAR B (white arrow) point away. Six telomere-associated repeat elements (TARE) and several 12-base SPE sites (bind P. falciparum SPE2 interacting protein 2, PfSIP2) are located between VAR genes and the telomere. RIF and STE genes are frequently found in the vicinity of VAR genes. Only one VAR gene is expressed at a time. Switches between expressed VAR genes confer allelic exclusion and antigenic variation. (D) In C. glabrata, EPA genes are organized in arrays and are the last protein encoding genes before the telomere. In the example shown in the figure, EPA3 and EPA2 are mostly silenced, while EPA1 is expressed. ESAG, expression site-associated gene; PfSIP2, P. falciparum SPE2 interacting protein 2.