Type 3 Fimbriae Encoded on Plasmids Are Expressed from a Unique Promoter without Affecting Host Motility, Facilitating an Exceptional Phenotype That Enhances Conjugal Plasmid Transfer

Horizontal gene transfer (HGT), the transmission of genetic material to a recipient that is not the progeny of the donor, is fundamental in bacterial evolution. HGT is often mediated by mobile genetic elements such as conjugative plasmids, which may be in conflict with the chromosomal elements of the genome because they are independent replicons that may petition their own evolutionary strategy. Here we study differences between type 3 fimbriae encoded on wild type plasmids and in chromosomes. Using known and newly characterized plasmids we show that the expression of type 3 fimbriae encoded on plasmids is systematically different, as MrkH, a c-di-GMP dependent transcriptional activator is not needed for strong expression of the fimbriae. MrkH is required for expression of type 3 fimbriae of the Klebsiella pneumoniae chromosome, wherefrom the fimbriae operon (mrkABCDF) of plasmids is believed to have originated. We find that mrkABCDFs of plasmids are highly expressed via a unique promoter that differs from the original Klebsiella promoter resulting in fundamental behavioral consequences. Plasmid associated mrkABCDFs did not influence the swimming behavior of the host, that hereby acquired an exceptional phenotype being able to both actively swim (planktonic behavior) and express biofilm associated fimbriae (sessile behavior). We show that this exceptional phenotype enhances the conjugal transfer of the plasmid.水平基因转移（Horizontal gene transfer, HGT）指遗传物质传递给非子代受体的过程，在细菌进化中具有核心意义。HGT通常由接合性质粒（conjugative plasmids）等可移动遗传元件（mobile genetic elements）介导，这类元件作为独立复制子（replicon）可自主追求自身的演化策略，因此往往与宿主基因组的染色体元件存在演化冲突。本研究聚焦野生型质粒与染色体编码的3型菌毛（type 3 fimbriae）之间的表达差异。通过已注释及新鉴定的质粒，我们发现质粒编码的3型菌毛的表达模式存在系统性差异：作为环二鸟苷酸（c-di-GMP）依赖的转录激活因子，MrkH对于该类菌毛的强表达并非必需。而在肺炎克雷伯菌（Klebsiella pneumoniae）的染色体中，3型菌毛的表达恰恰依赖MrkH；目前学界普遍认为，质粒携带的菌毛操纵子（fimbriae operon）mrkABCDF正是起源于该染色体位点。我们进一步发现，质粒携带的mrkABCDF可通过与原始肺炎克雷伯菌启动子（promoter）截然不同的独特启动子实现高效表达，这一差异带来了根本性的表型改变。携带该质粒的宿主菌株的游动行为（swimming behavior）未受影响，该宿主由此获得了兼具主动游动（浮游行为，planktonic behavior）与表达生物膜相关菌毛（固着行为，sessile behavior）的独特异常表型。我们证实，这种双重异常表型可显著增强质粒的接合转移（conjugal transfer）效率。