CHARACTERIZATION OF THE RHIZOBIUM-LEGUMINOSARUM BIOVAR PHASEOLI NIFA GENE, A POSITIVE REGULATOR OF NIF GENE-EXPRESSION

We report the isolation, mutational analysis and the nucleotide sequen ce of the Rhizobium leguminosarum bv. phaseoli nifA gene. Comparison o f the deduced amino acid sequence with other NifA sequences indicated the presence of the conserved central activator and the C-terminal DNA -binding domains. Nodules elicited by a R. leguminosarum bv. phaseoli nifA mutant were symbiotically ineffective. The expression of a nifA-g usA fusion was shown to be independent on the oxygen status of the cel l. We cloned the three nifH copies of R. leguminosarum bv. phaseoli an d determined the nucleotide sequence of their promoter regions. The ex pression of nifH-gusA fusions is induced under microaerobic conditions and is dependent on the presence of NifA.