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.
Gubler
M
M
Mikrobiologisches Institut, Eidgenössische Technische Hochschule, ETH-Zentrum, Zürich, Switzerland.
D001483:Base Sequence; D015152:Blotting, Northern; D002874:Chromosome Mapping; D002876:Chromosomes, Bacterial; D003001:Cloning, Molecular; D004742:Enhancer Elements, Genetic; D005798:Genes, Bacterial; D008969:Molecular Sequence Data; D009154:Mutation; D009586:Nitrogen Fixation / Q000235:genetics*; D010957:Plasmids; D011401:Promoter Regions, Genetic; D012045:Regulatory Sequences, Nucleic Acid; D012230:Rhizobiaceae / Q000235:genetics*; D001616:beta-Galactosidase / Q000235:genetics
D001616:beta-Galactosidase