Wirachman Erika S, Grossman Alan D
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America.
PLoS Genet. 2024 Dec 9;20(12):e1011417. doi: 10.1371/journal.pgen.1011417. eCollection 2024 Dec.
Premature expression of genes in mobile genetic elements can be detrimental to their bacterial hosts. Tn916, the founding member of a large family of integrative and conjugative elements (ICEs; aka conjugative transposons), confers tetracycline-resistance and is found in several Gram-positive bacterial species. We identified a transcription terminator near one end of Tn916 that functions as an insulator that prevents expression of element genes when Tn916 is integrated downstream from an active host promoter. The terminator blocked expression of Tn916 genes needed for unwinding and rolling circle replication of the element DNA, and loss of the terminator caused a fitness defect for the host cells. Further, we identified an element-encoded antiterminator (named canT for conjugation-associated antitermination) that is essential for transcription of Tn916 genes after excision of the element from the host chromosome. We found that the antiterminator is orientation-specific, functions with heterologous promoters and terminators, is processive and is most likely a cis-acting RNA. Insulating gene expression in conjugative elements that are integrated in the chromosome is likely a key feature of the interplay between mobile genetic elements and their hosts and appears to be critical for the function and evolution of the large family of Tn916-like elements.
移动遗传元件中基因的过早表达可能对其细菌宿主有害。Tn916是一大类整合性接合元件（ICEs；又称接合转座子）家族的创始成员，赋予四环素抗性，存在于几种革兰氏阳性细菌物种中。我们在Tn916的一端附近鉴定出一个转录终止子，它作为一种绝缘体发挥作用，当Tn916整合到活跃的宿主启动子下游时，可阻止元件基因的表达。该终止子阻断了元件DNA解旋和滚环复制所需的Tn916基因的表达，终止子的缺失导致宿主细胞出现适应性缺陷。此外，我们鉴定出一种元件编码的抗终止子（命名为canT，即与接合相关的抗终止），它对于元件从宿主染色体上切除后Tn916基因的转录至关重要。我们发现该抗终止子具有方向特异性，可与异源启动子和终止子一起发挥作用，具有持续性，很可能是一种顺式作用RNA。在整合于染色体中的接合元件中隔离基因表达可能是移动遗传元件与其宿主之间相互作用的一个关键特征，对于Tn916样元件大家族的功能和进化似乎至关重要。
