Multiple Chaperonins and Their Potential Roles in Rhizobia

Abstract

The Rhizobia are a paraphyletic group of gram-negative bacteria which are of enormous importance to agriculture, due to their ability to form nitrogen-fixing nodules on the roots of legumes. These organisms have a remarkable tendency to possess multiple copies of chaperonin genes, with up to seven copies being recorded—a record as far as bacterial genomes are concerned. The regulation of these multiple chaperonin genes is complex, suggesting that they may have roles in addition to protection from stresses that induce protein unfolding, and the proteins that they encode show little tendency to form mixed oligomers, again suggesting at least some specificity of function. Closely related bacteria that lack the ability to fix nitrogen often do not show this high chaperonin gene number. The conclusion is that the presence of the multiple chaperonins is likely to be related at least in part to the complex processes of nodule formation and nitrogen fixation, and some evidence exists to support this hypothesis.