Mutants in the nodFEL promoter of Rhizobium leguminosarum bv. viciae reveal a role of individual nucleotides in transcriptional activation and protein binding.

The highly conserved nod box sequence in the promoters of the inducible nodulation genes of rhizobia is required for transcription activation together with NodD, a LysR-type transcriptional regulator, and a flavonoid as a coinducer. DNA fragments containing nod box sequences form two binding complexes when crude preparations of Rhizobium leguminosarum bv. viciae are used: a NodD-dependent and an additional, NodD-independent complex.

The NodD protein does not bind to the promoters of inducible nodulation genes in extracts of bacteroids of Rhizobium leguminosarum biovar viciae.

In a previous study, we showed that in bacteroids, transcription of the inducible nod genes does not occur and expression of nodD is decreased by 65% (H. R. M.

Suppression of nodulation gene expression in bacteroids of Rhizobium leguminosarum biovar viciae.

The expression of nod genes of Rhizobium leguminosarum bv. viciae in nodules of Pisum sativum was investigated at both the translational and transcriptional levels. By using immunoblots, it was found that the levels of NodA, NodI, NodE, and NodO proteins were reduced at least 14-fold in bacteriods compared with cultured cells, whereas NodD protein was reduced only 3-fold.

Subcellular localization of the Rhizobium leguminosarum nodI gene product.

By the use of antibodies raised against a fusion protein of lacZ'-nodI (produced in Escherichia coli) which specifically react with NodI protein, it was shown that in wild-type Rhizobium leguminosarum biovar viciae NodI protein (i) is recovered with the cytoplasmic membrane fraction and (ii) is translated as part of the nodABCIJ operon. In addition, it was found that the bacterial chromosomal background strongly influences the expression of several nod genes.