ONE MEMBER OF A GROESL-LIKE CHAPERONIN MULTIGENE FAMILY IN BRADYRHIZOBIUM-JAPONICUM IS COREGULATED WITH SYMBIOTIC NITROGEN-FIXATION GENES

This report is concerned with the structural characterization and gene tic regulation of new bacterial groES and groEL chaperonin genes, and presents two novelties. The first is the discovery that the nitrogen f ixing soybean root nodule bacterium, Bradyrhizobium japonicum, unlike all other prokaryotes investigated so far, possesses a multigene famil y consisting of five very similar, though not identical, groESL-like g enes. The second novelty relates to the finding that these rive homolo gues are expressed to different degrees and, in particular, that one f amily member (namely groESL3) is induced by a mechanism that does not involve the well-known heat shock response. By contrast, the groESL3 g enes are co-regulated together with symbiotic nitrogen fixation genes, in that they are activated by the nitrogen fixation regulatory protei n NifA at low oxygen conditions and transcribed from a -24/-12 promote r by the sigma54 RNA polymerase. Two other members of the groESL gene family are apparently expressed constitutively at different levels, an d yet another one is strongly induced by high temperature. As an attra ctive hypothesis it follows that B.japonicum may modulate its cellular contents of GroES- and GroEL-like chaperonins in response to specific environmental conditions and physiological needs.