Fine-tuning of nif and fix gene expression by upstream activator sequences in Bradyrhizobium japonicum.

The significance of Bradyrhizobium japonicum upstream activator sequences (UASs) for differential NifA-mediated fix and nif gene expression was investigated by two means: (i) hybrid fixA- and fixB-lacZ fusions were constructed by transposing a nifH-UAS cartridge in front of their promoters; and (ii) B. japonicum mutants were generated carrying specific chromosomal deletions or UAS cartridge insertions within the fixA, fixB or nifH promoter-upstream regions. Expression of fixA was not affected, and expression of fixB decreased only to 42%, when the respective fixA and fixB promoter-upstream DNAs were deleted. This shows that in B. japonicum the NifA-dependent activation of at least the fixA promoter does not require the presence of a closely adjacent UAS. Deletion of the UASs in front of the nifH gene not only reduced the expression of nifH down to 2.5% but, surprisingly, also resulted in a reduction of the fixB mRNA level to less than 20%. This suggests that the nifH-UASs may exert a long-range effect on the expression of the 3-kb-distant fixBCX operon in nif cluster I or B. japonicum. Artificial transposition of the nifH-UASs in front of the fixA and fixB promoters strongly enhanced fixA and fixB expression.