Carbon Utilization and Regulation of Nitrogen Fixation Genes in Rhizobium meliloti. Kate Birkenhead. Department of Microbiology and National Food Biotechnology Centre University College, Cork, Ireland. Brian Noonan(1), William J. Reville(2), Bert Boesten(1), Sundaram S. Manian(1), and Fergal O’Gara(1). (1)Department of Microbiology and National Food Biotechnology Centre and (2)Electron Microscopy Unit, University College, Cork, Ireland.. MPMI 3:167-173. Accepted 8 December 1989. Copyright 1990 The American Phytopathological Society.

A dctB mutant (CM12) and a dctBD mutant (CM20) of Rhizobium meliloti displaying Fix^– and Fix⁺ phenotypes, respectively, were used to investigate the expression of nitrogen fixation genes during symbiosis. The levels of expression of the nifHDK promoter (P1), the fixABCX promoter (P2), and the nifA promoter (PnifA) were monitored during symbiosis and under free-living microaerobic conditions. The levels of expression in nodules formed by R. meliloti CM12 were less than 10% of the levels found in nodules formed by wild-type R. meliloti (strain CM2). Levels in nodules formed by CM20 were similar to those in wild-type cells. Under free-living microaerobic conditions, the level of expression of nitrogen fixation gene fusions in the wild-type R. meliloti CM2 varied strongly on different carbon sources used in the growth media. The highest levels of expression of PnifA and P1 were detected on dicarboxylic acids and on mannitol; the lowest were observed on glutamate. Glucose supported intermediate levels of expression. When mannitol was exploited as an alternative carbon source to support growth of the dct mutants, PnifA was found to be expressed at wild-type levels in both CM12 and CM20. In CM20, expression of PnifA resulted in wild-type levels of P1 expression. In CM12, however, this was not the case. The possibility of a regulatory link between the expression of R. meliloti nifHDK genes and the activity of the regulatory dct genes is discussed.