Autoregulation of fixK 2 gene expression in Bradyrhizobium japonicum
Reutimann, Luzia ; Mesa, Socorro ; Hennecke, Hauke
In: Molecular Genetics and Genomics, 2010, vol. 284, no. 1, p. 25-32
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- Several essential Bradyrhizobium japonicum genes for a symbiotic, nitrogen-fixing root-nodule symbiosis are positively controlled under micro-oxic conditions by the FixLJ-FixK2 regulatory cascade. Negative control is exerted by reactive oxygen species at the level of the FixK2 protein. Furthermore, we noticed that fixK 2 gene expression is increased in a fixK 2 mutant, suggesting that FixK2 in the wild type has a negative effect, directly or indirectly, on its own expression. To possibly understand this effect, the transcription pattern of the fixLJ-bll2758-fixK 2 gene region was examined more closely. While fixK 2 gene transcription is activated by FixJ, the bll2758 gene is transcribed from its own promoter in a FixK2-dependent manner, and there is no read-through transcription from bll2758 into fixK 2. The bll2758-encoded protein is predicted to be a stand-alone receiver domain of a response regulator, making it a prime candidate for exerting an inhibitory role on the expression of fixK 2. Transcriptome profiling of a bll2758 knock-out mutant revealed, however, that neither fixK 2 itself nor any of the known FixJ- and FixK2-dependent target genes is significantly affected in their expression. This precludes a role of the bll2758 product as a so-called FixT-like protein in the inhibition of FixLJ function, as was proposed for Sinorhizobium meliloti and Caulobacter crescentus. Instead, we rationalize that other transcription factors, whose genes are activated by FixK2, might be involved in the negative autoregulation of fixK 2 gene expression