Diversity of nif gene location and nitrogen fixation among root-associated Enterobacter and Klebsiella strains

Abstract

Our previous work showed that strains of dinitrogen fixing enterobacter and Klebsiella were found associated with the roots of uncultivated grasses in Finland more commonly than other species of diazotrophic bacteria. In this paper we compare E. agglomerans strains to K. pneumoniae and K. terrigena strains, and show that the E. agglomerans strains fall into two biogroups. The groups differ not only in the utilization of different carbon sources and other physiological characteristics such as the production of indole, but also in the physiology and genetics of nitrogenase activity. Biotype 1 (isolated from Achillea millefolium, Calamagrostis arundinacea, and Phleum pratense) showed active nitrogenase in atmospheric oxygen, whereas biotype 2 (from Phalaris arundinacea) resembled K. pneumoniae in that it was active at reduced oxygen pressure (pO2<-0.002) only. DNA of all strains showed positive hybridization with K. pneumoniae nifHDK genes (pSA30) but differed in the location of the genes. Biotype 1 strains of E. agglomerans carried nifHDK genes on large (105–125 Mdal) plasmids, whereas no plasmid was detected in biotype 2 or in the K. pneumoniae strains isolated from Agrostis stolonifera and Poa pratensis and K. terrigena strain isolated from Carex pallescens. The one K. terrigena strain (isolated from Ph. arundinacea) that was found to contain an indigenous plasmid (80 Mdal) did not carry nifHDK genes on this plasmid.