Hattman S, Ives J
Gene. 1984 Jul-Aug;29(1-2):185-98. doi: 10.1016/0378-1119(84)90179-3.
The mom gene of bacteriophage Mu encodes a DNA modification function. Expression of this modification requires the host Escherichia coli Dam (DNA-adenine methylase) function and the transacting phage Mu Dad function. The mom gene was subcloned into a variety of sites on plasmid pBR322. Insertions were made into the HincII and PvuI sites within the amp gene and into the ClaI site of the tet gene promoter. The only clones found were those in which the orientation of the mom gene prevents its transcription from the vector promoter(s), suggesting that constitutive expression of mom from a foreign promoter can occur independently of Dad function but is lethal for the cell. Employing S1 nuclease mapping, we have identified two Mu mRNA transcripts: (1) the gin transcript extends into the gin-mon intercistronic divide and terminates downstream from the BclI site; and (2) the mom transcript appears to initiate about 74 bp upstream from the BclI site, 12 bp downstream from a promoter-like sequence. Production of the mom transcript is dependent on the host Dam activity and on Dad transactivation. In contrast, the gin transcript is produced independently of Dam and Dad functions; the gin transcript may extend into the mom gene, but it appears to be either degraded at the 3' end or differentially terminated. We propose that regulation of mom gene transcription involves both positive and negative regulatory proteins, and that binding of the Dad protein (a "late" Mu protein) is required for transcription initiation by the host RNA polymerase. However, Dad protein action may be inhibited by prior binding of a repressor to the mom operator, located farther upstream. We propose that this repressor (encoded by a phage or host gene) binds to the operator only when there is no active Dam enzyme present, i.e., when there is no methylation of (or methylase binding to) the G-A-T-C sites within the mom operator.
噬菌体Mu的mom基因编码一种DNA修饰功能。这种修饰的表达需要宿主大肠杆菌Dam（DNA腺嘌呤甲基化酶）功能和反式作用的噬菌体Mu Dad功能。mom基因被亚克隆到质粒pBR322的多个位点。插入到amp基因内的HincII和PvuI位点以及tet基因启动子的ClaI位点。唯一发现的克隆是那些mom基因的方向阻止其从载体启动子转录的克隆，这表明从外源启动子组成型表达mom可以独立于Dad功能发生，但对细胞是致死的。利用S1核酸酶图谱，我们鉴定出两种Mu mRNA转录本：（1）gin转录本延伸到gin - mon顺反子间区域并在BclI位点下游终止；（2）mom转录本似乎在BclI位点上游约74 bp处起始，在一个类似启动子序列下游12 bp处。mom转录本 的产生依赖于宿主Dam活性和Dad反式激活。相反，gin转录本的产生独立于Dam和Dad功能；gin转录本可能延伸到mom基因中，但它似乎在3'端被降解或差异终止。我们提出mom基因转录的调控涉及正调控和负调控蛋白，并且宿主RNA聚合酶转录起始需要Dad蛋白（一种“晚期”Mu蛋白）的结合。然而，Dad蛋白的作用可能被阻遏物先前与位于更上游的mom操纵子的结合所抑制。我们提出这种阻遏物（由噬菌体或宿主基因编码）仅在没有活性Dam酶存在时，即当mom操纵子内的G - A - T - C位点没有甲基化（或甲基化酶结合）时，才与操纵子结合。
