Kataoka M, Seki T, Yoshida T
International Center of Cooperative Research in Biotechnology, Faculty of Engineering, Osaka University, Japan.
J Bacteriol. 1991 Dec;173(24):7975-81. doi: 10.1128/jb.173.24.7975-7981.1991.
pSN22 is an 11-kb multicopy plasmid from Streptomyces nigrifaciens which is being studied in Streptomyces lividans. A segment of about 7 kb of pSN22 contains five genes involved in conjugation. Three of them, traA, traB, and traR, are essential for plasmid transfer and for the mobilization of chromosomal markers (fertility), while the remaining two genes, spdA and spdB, merely enhance the efficiency of plasmid transfer, resulting in the formation of larger pocks. In vitro promoter-probing experiments identified a 550-bp BglII-SmaI DNA fragment with promoter activity in both orientations; Northern (RNA blot) hybridization identified corresponding divergent transcripts of 1 and 5.2 kb for traR and the traA-traB-spdB operon, respectively. The traR gene product repressed its own transcription and also the transcription of the traA-traB-spdB operon. Plasmids containing a functional traB gene could not "survive" without traR being present in the same cell either in cis or in trans, presumably because unregulated expression of traB is lethal to the host. Plasmids with a functional traA gene but without traR had a low transformation efficiency and inhibited the growth of host cells.
pSN22是一种来自黑色链霉菌的11千碱基对的多拷贝质粒，目前正在淡紫链霉菌中进行研究。pSN22约7千碱基对的一段包含五个参与接合作用的基因。其中三个基因，traA、traB和traR，对于质粒转移和染色体标记的动员（育性）至关重要，而其余两个基因，spdA和spdB，仅提高质粒转移效率，导致形成更大的噬菌斑。体外启动子探测实验鉴定出一个550碱基对的BglII-SmaI DNA片段，其在两个方向上均具有启动子活性；Northern（RNA印迹）杂交分别鉴定出traR以及traA-traB-spdB操纵子对应的1千碱基对和5.2千碱基对的反向转录本。traR基因产物抑制其自身的转录以及traA-traB-spdB操纵子的转录。含有功能性traB基因的质粒，若traR不在同一细胞中顺式或反式存在，则无法“存活”，推测是因为traB的无调控表达对宿主具有致死性。具有功能性traA基因但没有traR的质粒转化效率低，并抑制宿主细胞的生长。
