Wu G J, Cannon R E
J Biol Chem. 1986 Sep 25;261(27):12633-42.
Both VARNA genes in the adenovirus genome are transcribed by host DNA-dependent RNA polymerase III. Nevertheless, late in infection the mass ratio of VARNA2 to VARNA1 in adenovirus-infected human KB or HeLa cells is about 1:40. This difference is most likely due to differences in promoter strength which may be attributed to sequence differences in the control regions of the two genes. To investigate this possibility, the two genes were cloned into separate plasmid molecules and their transcription efficiencies were compared in vitro. The VARNA2 gene was transcribed in vitro at least 50 times less efficiently than the VARNA1 gene. The competing strengths of the two genes were similar, which suggests that the B block sequence in the VARNA2 gene may be fully functional for sequestering factors. Therefore, the major difference must reside in the region upstream of the B block. By analyzing the hybrid RNAs transcribed from a hybrid gene and 27 fused genes in which only the control region of the VARNA1 gene was fused to various lengths of the 5'-flanking and the coding regions of the VARNA2 gene, one major and one minor termination site, which caused premature termination, were revealed downstream of the A block sequence and in the 5'-flanking region of the VARNA2 gene, respectively. The presence of termination sequences, a suboptimal interblock spacing, and an altered A block sequence in the control region may result in a weaker promoter in the VARNA2 gene.
腺病毒基因组中的两个VARNA基因均由宿主DNA依赖性RNA聚合酶III转录。然而，在感染后期，腺病毒感染的人KB或HeLa细胞中VARNA2与VARNA1的质量比约为1:40。这种差异很可能是由于启动子强度的差异，这可能归因于两个基因控制区域的序列差异。为了研究这种可能性，将这两个基因克隆到不同的质粒分子中，并在体外比较它们的转录效率。VARNA2基因在体外的转录效率比VARNA1基因至少低50倍。这两个基因的竞争强度相似，这表明VARNA2基因中的B块序列在隔离因子方面可能具有完全功能。因此，主要差异必定存在于B块上游区域。通过分析从一个杂交基因和27个融合基因转录的杂交RNA，其中仅将VARNA1基因的控制区域与VARNA2基因的各种长度的5'侧翼和编码区域融合，分别在A块序列下游和VARNA2基因的5'侧翼区域揭示了一个主要和一个次要的终止位点，这导致了提前终止。控制区域中终止序列的存在、次优的块间间距以及A块序列的改变可能导致VARNA2基因中的启动子较弱。
