Prakash O, Guntaka R V, Sarkar N H
Gene. 1983 Aug;23(2):117-30. doi: 10.1016/0378-1119(83)90043-4.
The long terminal repeat (LTR) of C3H murine mammary tumor virus (MuMTV) is approx. 1.3 kb long. HaeIII digestion of a cloned PstI fragment containing the left-end LTR generated four fragments of sizes 0.56, 0.41, 0.34 and 0.14 kb, one of which (0.41 kb) had a promoter activity in Escherichia coli. This was demonstrated by replacing the bacterial promoter for the neomycin-resistance (NmR) gene in the plasmid pKC56 with the HaeIII fragments. Only the 0.41-kb fragment that contains sequences from the U3 region of the LTR was found to contain a promoter, as shown by the expression of the drug-resistance phenotype in the recombinant plasmid. The strength of this promoter was comparable to or greater than that found with the parental NmR gene promoter. S1 nuclease mapping of the NmR gene transcript indicated that the initiation of this transcript occurred within the 0.41-kb LTR fragment from a site approx. 10 bp upstream from the 3' end. A comparison of the known DNA sequences in the MuMTV LTR with those found in bacterial promoters revealed that a 'Pribnow box', the initiation signal for the prokaryotic promoters, is present in the 0.41-kb LTR fragment upstream from the initiation site. Furthermore, in a recombinant plasmid that contained the complete LTR the same promoter sequences appeared to be involved in the initiation of RNA transcription. The 0.34-kb LTR fragment, which contains sequences derived from the U3 and U5 regions of the LTR, did not possess promoter activity in E. coli. However, it was found to induce deletions of adjacent plasmid DNA sequences. The deletions were specifically initiated from the downstream end of the LTR-fragment insert. The presence of a prokaryotic promoter in the MuMTV LTR, together with the observation that certain LTR sequences can induce deletions, analogous to those caused by transposable elements, in recombinant plasmids suggest that the MuMTV LTR may have evolved from such elements.
C3H小鼠乳腺肿瘤病毒（MuMTV）的长末端重复序列（LTR）约1.3 kb长。对含有左端LTR的克隆PstI片段进行HaeIII酶切产生了大小分别为0.56、0.41、0.34和0.14 kb的四个片段，其中一个（0.41 kb）在大肠杆菌中具有启动子活性。通过用HaeIII片段替换质粒pKC56中新霉素抗性（NmR）基因的细菌启动子证明了这一点。只有包含来自LTR的U3区域序列的0.41 kb片段被发现含有启动子，重组质粒中耐药表型的表达证明了这一点。该启动子的强度与亲本NmR基因启动子相当或更强。对NmR基因转录本进行S1核酸酶作图表明，该转录本的起始发生在0.41 kb LTR片段内，起始位点位于3'端上游约10 bp处。将MuMTV LTR中已知的DNA序列与细菌启动子中的序列进行比较发现，原核启动子的起始信号“Pribnow框”存在于起始位点上游的0.41 kb LTR片段中。此外，在含有完整LTR的重组质粒中，相同的启动子序列似乎参与了RNA转录的起始。包含来自LTR的U3和U5区域序列的0.34 kb LTR片段在大肠杆菌中不具有启动子活性。然而，发现它会诱导相邻质粒DNA序列的缺失。缺失是从LTR片段插入物的下游末端特异性起始的。MuMTV LTR中存在原核启动子，以及某些LTR序列可在重组质粒中诱导类似于转座元件引起的缺失这一观察结果表明，MuMTV LTR可能是从这类元件进化而来的。
