Wills N M, Gesteland R F, Atkins J F
Howard Hughes Medical Institute, University of Utah, Salt Lake City 84112.
EMBO J. 1994 Sep 1;13(17):4137-44. doi: 10.1002/j.1460-2075.1994.tb06731.x.
Retroviruses whose gag and pol genes are in the same reading frame depend upon approximately 5% read-through of the gag UAG termination codon to make the gag-pol polyprotein. For murine leukemia virus, this read-through is dependent on a pseudoknot located eight nucleotides 3' of the UAG. Other retroviruses whose gag and pol genes are in the same frame can potentially form similar pseudoknots 3' of their UAG codons. Beyond the similar secondary structures, there is strong sequence conservation in the spacer region and in loop 2 of the pseudoknots. The detrimental effects of substitutions of several of these conserved spacer and loop 2 nucleotides in the murine leukemia virus sequence show their importance for the read-through process. The importance of specific nucleotides in loop 2 of the pseudoknot contrasts with the flexibility of sequence in loop 2 of the most intensively studied frameshift-promoting pseudoknot which occurs in infectious bronchitis virus. Two nucleotides in loop 2 of the murine leukemia virus pseudoknot, which were shown to be important by mutagenic analysis, display hypersensitivity to the single-strand specific nuclease, S1. They are likely to be particularly accessible or are in an unusually reactive conformation.
其gag和pol基因处于同一阅读框的逆转录病毒，要通过gag基因的UAG终止密码子约5%的通读才能产生gag-pol多聚蛋白。对于鼠白血病病毒而言，这种通读依赖于位于UAG下游8个核苷酸处的假结。其他gag和pol基因处于同一阅读框的逆转录病毒，在其UAG密码子下游也可能形成类似的假结。除了相似的二级结构外，假结的间隔区和环2中存在很强的序列保守性。在鼠白血病病毒序列中，这些保守的间隔区和环2核苷酸的替换所产生的有害影响，表明了它们在通读过程中的重要性。假结环2中特定核苷酸的重要性，与传染性支气管炎病毒中研究最为深入的移码促进假结环2序列的灵活性形成对比。通过诱变分析表明，鼠白血病病毒假结环2中的两个核苷酸对单链特异性核酸酶S1表现出超敏感性。它们可能特别易于接近，或者处于异常活跃的构象中。
