What does a two-component regulator do when it isn’t phosphorylated?

Bacterial Genetics and Genomics, Chapter 8, Discussion topic 2.

As described in Chapter 8 of Bacterial Genetics and Genomics, a two-component regulatory system has a sensory protein in the membrane that autophorphorylates when triggered by its sensory signal and a regulatory protein that is phosphorylated to do its job adjusting gene expression.  In this way, something sensed by the cell at its surface can influence the genes that are turned on or off and therefore respond to the stimulus.

Since this process is only required by the bacterial cell some of the time, we would not expect the components of a two-component regulatory system to be essential for the life of the cell. But, there are some studies that are showing essential regulatory protein components, suggesting another role for these proteins.

In Sinorhizobium meliloti, a soil bacterium particularly associated with legumes, the roles of the CenK histidine kinase sensor and the CenR response regulator were assessed. The CenKR two-component regulatory system has been shown to regulate srlA, a gene for a thioredoxin-like protein that is associated with high salt tolerance. The srlA gene is activated by the CenKR system, with CenR binding to an inverted repeat region in slrA’s promoter.

Photograph of the root nodules formed due to S. meliloti.

Interestingly, CenR is essential in S. meliloti, with the bacterial cells unable to survive the absence of this regulatory protein. However, its partner in the two-component regulatory system, CenK, is not essential. This suggested that CenR was either doing something important without being phosphorylated by CenK or was getting its phosphorylation elsewhere.

Using some clever single amino acid substitutions in the CenR protein, Freire et al (2024) were able to show that the essential function of this regulator did not require the protein to be phosphorylated.

Phosphorylation is required for regulation of SrlA, therefore this is not the gene that is involved in the essential function of CenR. Identifying this will require some more research into the regulatory networks in S. meliloti. This illustrates the importance of researching gene functions in the lab and not making too many assumptions about function based on the annotation of the sequences. Yes, there is a two-component regulatory system, but the regulatory protein part of this system may have another job in the cell.