Multicopy Plasmids Carrying the Klebsiella pneumoniae nifA Gene Enhance Rhizobium meliloti Nodulation Competitiveness on Alfalfa. Juan Sanjuan. Departamento de Microbiologia, Estación Experimental del Zaidin, Consejo Superior de Investigaciones Cientificas, E-18080 Granada, Spain. Jose Olivares. Departamento de Microbiologia, Estación Experimental del Zaidin, Consejo Superior de Investigaciones Cientificas, E-18080 Granada, Spain.. MPMI 4:365-369. Accepted 25 March 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991.

Three regulatory mutants of Rhizobium meliloti (NifA‾, NtrA‾, or NtrC‾) showed delayed nodulation and were less competitive when coinoculated with the wild-type strain 2011 on alfalfa roots. Whereas the defect for the NtrA‾ and NtrC‾ strains might be due to a reduced level of nod gene expression under nitrogen-limiting conditions, we found that the NifA‾ strain expressed a nodC-lacZ fusion at wild-type levels under all growing conditions tested. However, the reduced nodulation competitiveness shown by this NifA? strain does not seem to be due to its Fix‾ phenotype. When plasmid pCK3, carrying the Klebsiella pneumoniae nifA gene, was transferred into the three regulatory mutants, wild-type nodulation competitiveness was only restored in the case of the nifA mutant strain. However, such complementation did not restore symbiotic nitrogen-fixing ability. Furthermore, the introduction of multicopy plasmids containing the cloned nifA gene from K. pneumoniae into four wild-type R. meliloti strains resulted in a generalized increase of their respective competitive abilities.