Recurrent paralogy in the evolution of archaeal chaperonins

Chaperonins are multisubunit double-ring complexes that mediate the folding of nascent proteins [1,2], In bacteria, chaperonins are homo-oligomeric an d are composed of seven-membered rings. Eukaryotic and most archaeal chaper onin rings are eight-membered and exhibit varying degrees of hetero-oligome rism [3,4]. We have cloned and sequenced seven new genes encoding chaperoni n subunits from the crenarchaeotes Sulfolobus solfataricus, S. acidocaldari us, S. shibatae and Desulfurococcus mobilis. Although some archaeal genomes possess a single chaperonin gene, most have two. We describe a third chape ronin-encoding gene (TF55-gamma) from two Sulfolobus species; phylogenetic analyses indicate that the gene duplication producing TF55-gamma occurred w ithin crenarchaeal evolution. The presence of TF55-gamma in Sulfolobus corr elates with their unique nine-membered chaperonin rings. Duplicate genes (p aralogs) for chaperonins within archaeal genomes very often resemble each o ther more than they resemble chaperonin genes from other archaea. Our phylo genetic analyses suggest multiple independent gene duplications - at least seven among the archaea examined. The persistence of paralogous genes for c haperonin subunits in multiple archaeal lineages may involve a process of c o-evolution, where chaperonin subunit heterogeneity changes independently o f selection on function.