Two of the Three GroEL Homologues in Rhizobium Leguminosarum Are Dispensable for Normal Growth

Although many bacteria contain only a single groE operon encoding the essential chaperones GroES and GroEL, examples of bacteria containing more than one groE operon are common. The root-nodulating bacterium Rhizobium leguminosarum contains at least three operons encoding homologues to Escherichia coli GroEL, referred to as Cpn60.1, Cpn60.2 and Cpn60.3, respectively. We report here a detailed analysis of the requirement for and relative levels of these three proteins. Cpn60.1 is present at higher levels than Cpn60.2, and Cpn60.3 protein could not be detected under any conditions although the cpn60.3 gene is transcribed under anaerobic conditions. Insertion mutations could not be constructed in cpn60.1 unless a complementing copy was present, showing that this gene is essential for growth under the conditions used here. Both cpn60.2 and cpn60.3 could be inactivated with no loss of viability, and a double cpn60.2 cpn60.3 mutant was also constructed which was fully viable. Thus only Cpn60.1 is required for growth of this organism.