Department of Oral Biology, The State University of New York at Buffalo, New York 14214, USA.
Mol Microbiol. 2011 Nov;82(4):851-64. doi: 10.1111/j.1365-2958.2011.07853.x. Epub 2011 Oct 18.
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 (CsrA(Bb); 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 CsrA(Bb) on FlaB synthesis and the mechanism involved by analysing two mutants, csrA(Bb)(-) (a deletion mutant of csrA(Bb)) and csrA(Bb)(+) (a mutant conditionally overexpressing csrA(Bb)). We found that FlaB accumulation was significantly inhibited in csrA(Bb)(+) but was substantially increased in csrA(Bb)(-) . In contrast, the levels of other flagellar proteins remained unchanged. Cryo-electron tomography and immuno-fluorescence microscopic analyses revealed that the altered synthesis of CsrA(Bb) 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 CsrA(Bb) 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 CsrA(Bb) specifically regulates the periplasmic flagellar synthesis by inhibiting translation initiation of the flaB transcript.
莱姆病螺旋体伯氏疏螺旋体缺乏其他细菌中普遍存在的鞭毛蛋白合成的转录级联控制。相反，它依赖于一种转录后机制来控制其鞭毛合成。这种控制的潜在机制仍然难以捉摸。最近的一项研究报告称，BB0184（CsrA（Bb）；碳储存调节剂 A 的同源物）水平的增加极大地抑制了主要鞭毛蛋白 FlaB 的积累。在本报告中，我们通过分析两个突变体 csrA（Bb）（-）（csrA（Bb）的缺失突变体）和 csrA（Bb）（+）（csrA（Bb）条件性过表达的突变体），解析了 CsrA（Bb）对 FlaB 合成的调控作用及其涉及的机制。我们发现 csrA（Bb）（+）中 FlaB 的积累显著受到抑制，但 csrA（Bb）（-）中显著增加。相比之下，其他鞭毛蛋白的水平保持不变。冷冻电子断层扫描和免疫荧光显微镜分析显示，这两个突变体中 CsrA（Bb）的合成改变特异性地影响了鞭毛丝的长度。flaB 转录物的前导序列包含两个保守的 CsrA 结合位点，其中一个位点与 Shine-Dalgarno 序列重叠。我们发现 CsrA（Bb）通过这两个结合位点结合到 flaB 转录物上，这种结合抑制了 FlaB 在翻译水平上的合成。总之，我们的结果表明 CsrA（Bb）通过抑制 flaB 转录物的翻译起始来特异性地调节周质鞭毛的合成。
