AN INSERTIONAL POINT MUTATION INACTIVATES NOLR REPRESSOR IN RHIZOBIUM-MELILOTI 1021

In the majority of Rhizobium meliloti isolates, nod gene expression is controlled by NolR but this is not the case in a few strains includin g the widely used laboratory strain 1021. In 1021, the lack of NolR fu nction was shown to be due to a single insertional mutation in the C-t erminal coding sequence which abolished the DNA-binding ability, thoug h the helix-turn-helix motif remained intact. This indicates that the C-terminal part of the protein is also essential for DNA binding. We c onclude that in this species, control of nod gene expression involves NolR and strain 1021 represents an exception in which the NolR functio n was lost by a single event.