Flower A M, McHenry C S
Department of Biochemistry, Biophysics and Genetics, University of Colorado Health Sciences Center, Denver 80262.
Proc Natl Acad Sci U S A. 1990 May;87(10):3713-7. doi: 10.1073/pnas.87.10.3713.
The tau and gamma subunits of DNA polymerase III holoenzyme are both products of the dnaX gene. Since tau and gamma are required as stoichiometric components of the replicative complex, a mechanism must exist for the cell to coordinate their synthesis and ensure that both subunits are present in an adequate quantity and ratio for assembly. We have proposed that gamma is produced by a translational frameshift event. In this report, we describe the use of dnaX-lacZ fusions in all three reading frames to demonstrate that gamma, the shorter product of dnaX, is generated by ribosomal frameshifting to the -1 reading frame of the mRNA within an oligo(A) sequence that is followed by a sequence predicted to form a stable secondary structure. Immediately after frameshifting a stop codon is encountered, leading to translational termination. Mutagenesis of the oligo(A) sequence abolishes frameshifting, and partial disruption of the predicted distal secondary structure severely impairs the efficiency. Comparison of the expression of lacZ fused to dnaX distal to the site of frameshifting in the -1 and 0 reading frames indicates that the efficiency of frameshifting is approximately 40%.
DNA聚合酶III全酶的τ亚基和γ亚基均为dnaX基因的产物。由于τ和γ是复制复合物的化学计量组分，细胞必定存在一种机制来协调它们的合成，并确保两个亚基都以适当的数量和比例存在以进行组装。我们曾提出γ是由翻译移码事件产生的。在本报告中，我们描述了在所有三个阅读框中使用dnaX-lacZ融合体，以证明dnaX的较短产物γ是通过核糖体移码至mRNA的-1阅读框而产生的，该移码发生在一个寡聚（A）序列内，该序列之后是一个预计会形成稳定二级结构的序列。移码后立即遇到一个终止密码子，导致翻译终止。寡聚（A）序列的诱变消除了移码，并且预测的远端二级结构的部分破坏严重损害了移码效率。在-1和0阅读框中，对移码位点远端与dnaX融合的lacZ的表达进行比较表明，移码效率约为40%。
