In Cereibacter sphaeroides, expression of flagellin FlaA is posttranscriptionally controlled by the 5′ untranslated region of its mRNA and the regulatory proteins FlaF, FlbT and FlbR

In bacteria, flagellar filaments consist of thousands of flagellin subunits, making their assembly energetically costly. Flagellin synthesis is tightly regulated and in alpha-proteobacteria, this is achieved through a posttranscriptional mechanism mediated by the FlbT and FlaF proteins, although, their specific roles appear to vary among different species. In this study, we show that in Cereibacter sphaeroides in addition to FlaF and FlbT, a third regulatory protein, FlbR, posttranscriptionally regulate the FlaA flagellin synthesis. In the absence of any of these proteins, a severe reduction of FlaA is observed compared to the wild-type strain. This effect does not correlate with the expression of a reporter gene under the control of the flaA promoter but a marked decrease in flaA mRNA levels was observed. Because FlaA synthesis was restored by deleting the 5′-UTR of the flaA mRNA, we propose that this region is presumably the target of FlaF, FlbT and FlbR to control flaA mRNA stability. Furthermore, we show that FlbT by itself binds the flaA 5′-UTR. This posttranscriptional regulation is likely relevant during flagellar morphogenesis, given that preventing the assembly of early flagellar structures results in a reduction of FlaA, which is reversed upon deletion of the 5′-UTR. However, the negative role of the 5′-UTR was not reproduced when this region was placed upstream of a reporter gene, indicating that other regions of the flaA mRNA may also contribute to its regulation. • FlaA is posttranscriptionally regulated by FlaF and FlbT and the new FlbR protein. • The 5′-UTR of the flaA mRNA, negatively impacts the expression FlaA. • FlaF, FlbT and FlbR are required to counteract the negative effect of the 5′-UTR. • FlbT directly binds the flaA 5′-UTR. • FlaA expression is linked to flagellar assembly via the 5′-UTR.