Oxygen sensitivity and metal ion-dependent transcriptional activation by NIFA protein from Rhizobium leguminosarum biovar trifolii.

The NIFA protein from Rhizobium leguminosarum biovar trifolii (R. trifolii) strain ANU843 lacks an N-terminal domain present in homologous NIFA proteins from other diazotrophs. The R. trifolii nifA gene product is unstable when expressed in Escherichia coli under both aerobic and microaerobic conditions. Stability is increased by fusion of additional amino acids to the N-terminus of the protein or by expression of nifA in sno mutant (presumed protease deficient) strains of E. coli. Transcriptional activation in vivo by R. trifolii NIFA decreases under aerobic growth conditions, or when cultures are depleted of metal ions. In sno mutant strains this decrease in activity reflects a loss of specific activity rather than proteolytic degradation, implying that R. trifolii NIFA requires metal ions for activity and is oxygen sensitive. The addition of 30 amino acids to the amino-terminus of R. trifoli NIFA results in an oxygen-tolerant protein, with metal ion-dependent activity. Metal ions are therefore not only required for oxygen sensing by R. trifolii NIFA but may play an additional role in determining NIFA structure or activity.