Journal of General Microbiology (1988), 134, 61 1-618. Printed in Great Britain 61 1 Effects of Oxygen Levels on the Transcription of nifand gln Genes in Bradyrhizobium japonicum By THOMAS H. ADAMS*t AND BARRY K. CHELMS MSUlDOE Plant Research Laboratory and the Department of Microbiology, Michigan State University, East Lansing, Michigan 48824, USA (Received 13 July 1987; revised 28 September 1987) The transciption of genes that function in N2 fixation (nif) and nitrogen assimilation (gln11) in Bradyrhizobiurn japonicum is coordinately induced in response to O2 limitation as well as symbiotic development. We have determined the relative steady-state mRN A levels for the nzfH, nfDK and glnll transcription units in bradyrhizobial cells grown under a variety of levels of aerobiosis and in cells isolated from soybean root nodules. All three transcripts are found in cells grown in a rich medium sparged with O2 concentrations of 5 % (v/v) or less. This expression is qualitatively similar to that observed for Bradyrhizobium during symbiotic development. Potential physiological mechanisms for the coordinate control of these genes are discussed. INTRODUCTION Rhizobia and bradyrhizobia reduce atmospheric N2 in symbiotic association with their leguminous host plants. Development of symbiotic N2 fixation is a complex process requiring the coordinate differentiation of both plant and bacterial cells. This differentiation results in the formation of morphologically and functionally distinct N,-fixing endosymbiont bacteria termed ‘bacteroids’. Very little of the ammonia produced from N, reduction by bacteroids is used to support bacterial growth (Brown & Dilworth, 1975; Stripf & Werner, 1978; Werner et al., 1980): most of it is exported (Bergerson & Turner, 1967) to the plant where it is assimilated (Miflin & Lea, 1976). The plant supplies reduced carbon compounds to the bacteria to meet the high energy demands of N, fixation and cell maintenance. Nitrogen metabolism during asymbiotic N2 fixation by bradyrhizobia is similar to that observed in bacteroids, as most of the fixed nitrogen is not used to support growth, but is instead exported (O’Gara & Shanmugan, 1976; Bergerson & Turner, 1978; Ludwig, 1980a, b). For at least some bradyrhizobial strains this failure to assimilate fixed nitrogen is partly due to an apparent decrease in glutamine synthetase (GS) activity (Ludwig, 1980a, b; Bergerson & Turner, 1978). Regulation of GS activity in rhizobia and bradyrhizobia is complicated by the fact that these bacteria contain two isoforms of the enzyme, GSI and GSII (Darrow & Knotts, 1977). The two distinct nitrogen-assimilatory enzymes are encoded by separate genes (Somerville & Kahn, 1983; Carlson & Chelm, 1986), and transcription of the genes is differentially regulated. Transcription of the gene encoding GSI (glnA) is constant under all conditions (Carlson et al., 1985). This enzyme is regulated post-translationally via adenylylation, such that activity decreases during growth in nitrogen-rich media (Bishop et al., 1976; Ludwig, 1978). In contrast, no post-translational modification of GSII has been observed. Transcription of the gene encoding GSII (glnll) under aerobic growth conditions is controlled by a mechanism that resembles the Ntr system (Carlson et al., 1987) of enteric bacteria (Magasanik, 1982; Gussin et al., 1986); growth conditions that limit nitrogen source activate transcription of the gln11 gene. t Present address: Department of Genetics, University of Georgia, Athens, Georgia 30602, USA. 3 This paper is dedicated to the memory of Barry Chelm. Abbreviation : GS, glutamine synthetase. 0001-4296 0 1988 SGM