Sinorhizobium meliloti nfe (nodulation formation efficiency) genes exhibittemporal and spatial expression patterns similar to those of genes involved in symbiotic nitrogen fixation

The life genes (nfeA, nfeB, and nfeD) are involved in the nodulation effici ency and competitiveness of the Sinorhizobium meliloti strain GR4 on alfalf a roots. The nfeA and nfeB genes are preceded by functional nif consensus s equences and NifA binding motifs, Here, we determined the temporal and spat ial expression patterns of the nfe genes in symbiosis with alfalfa, Transla tional fusions of the nfe promoters with the gusA gene and reverse transcri ption-polymerase chain reaction analyses indicate that they are expressed a nd translated within mature nitrogen-fixing modules and not during early st eps of nodule development. Within the nodules the three nfe genes exhibit a spatial expression pattern similar to that of genes involved in symbiotic nitrogen fixation. We show that nfeB and nfeD genes are expressed not only from their own promoters but also from the upstream nfe promoter sequences. Furthermore, with the use of specific antibodies the NfeB and NfeD protein s were detected within the root nodule bacteroid fraction. Finally, NfeB wa s immunolocalized in the bacteroid cell membrane whereas NfeD was detected in the bacteroid cytoplasm.