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Carbon storage regulator A (CsrABb) is a repressor of Borrelia burgdorferi flagellin protein FlaB

The Lyme disease spirochete Borrelia burgdorferi lacks the transcriptional cascade control of flagellar protein synthesis common to other bacteria. Instead, it relies on a post‐transcriptional mechanism to control its flagellar synthesis. The underlying mechanism of this control remains elusive. A recent study reported that the increased level of BB0184 (CsrABb; a homologue of carbon storage regulator A) substantially inhibited the accumulation of FlaB, the major flagellin protein of B. burgdorferi. In this report, we deciphered the regulatory role of CsrABb on FlaB synthesis and the mechanism involved by analysing two mutants, csrABb‐ (a deletion mutant of csrABb) and csrABb+ (a mutant conditionally overexpressing csrABb). We found that FlaB accumulation was significantly inhibited in csrABb+ but was substantially increased in csrABb‐. In contrast, the levels of other flagellar proteins remained unchanged. Cryo‐electron tomography and immuno‐fluorescence microscopic analyses revealed that the altered synthesis of CsrABb in these two mutants specifically affected flagellar filament length. The leader sequence of flaB transcript contains two conserved CsrA‐binding sites, with one of these sites overlapping the Shine–Dalgarno sequence. We found that CsrABb bound to the flaB transcripts via these two binding sites, and this binding inhibited the synthesis of FlaB at the translational level. Taken together, our results indicate that CsrABb specifically regulates the periplasmic flagellar synthesis by inhibiting translation initiation of the flaB transcript.