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首页> 外文期刊>Journal of Molecular Biology >DNA sequences in gal operon override transcription elongation blocks.
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DNA sequences in gal operon override transcription elongation blocks.

机译：gal操纵子中的DNA序列覆盖了转录延伸块。

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The DNA loop that represses transcription from galactose (gal) promoters is infrequently formed in stationary-phase cells because the concentration of the loop architectural protein HU is significantly low at that state, resulting in expression of the operon in the absence of the gal inducer D-galactose. Unexpectedly, transcription from the gal promoters under these conditions overrides physical block because of the presence of the Gal repressor bound to an internal operator (O(I)) located downstream of the promoters. We have shown here that although a stretch of pyrimidine residues (UUCU) in the RNA:DNA hybrid located immediately upstream of O(I) weakens the RNA:DNA hybrid and favors RNA polymerase (RNAP) pausing and backtracking, a stretch of purines (GAGAG) in the RNA present immediately upstream of the pause sequence in the hybrid acts as an antipause element by stabilizing the RNA:DNA duplex and preventing backtracking. This facilitates forward translocation of RNAP, including overriding of the DNA-bound Gal repressor barrier at O(I). When the GAGAG sequence is separated from the pyrimidine sequence by a 5-bp DNA insertion, RNAP backtracking is favored from a weak hybrid to a more stable hybrid. RNAP backtracking is sensitive to Gre factors, D-galactose, and antisense oligonucleotides. The ability of a native DNA sequence to override transcription elongation blocks in the gal operon uncovers a previously unknown way of regulating gal metabolism in Escherichia coli. It also explains the synthesis of gal enzymes in the absence of inducer for biosynthetic reactions.
机译：在固定相细胞中很少会形成抑制半乳糖（gal）启动子转录的DNA环，因为在该状态下环结构蛋白HU的浓度非常低，导致在没有gal诱导剂D的情况下操纵子的表达。 -半乳糖。出乎意料的是，在这些条件下从gal启动子的转录优先于物理阻断，因为存在与启动子下游的内部​​操纵子（O（I））结合的Gal阻遏物。我们在这里表明，尽管位于O（I）上游的RNA：DNA杂种中的嘧啶残基（UUCU）片段会削弱RNA：DNA杂种并有利于RNA聚合酶（RNAP）暂停和回溯，但一部分嘌呤（通过稳定RNA：DNA双链体并防止回溯，杂合体中暂停序列的紧靠上游的RNA中的GAGAG（GAGAG）可作为抗暂停元素。这促进了RNAP的正向移位，包括覆盖了O（I）处DNA结合的Gal阻遏物屏障。当通过5 bp DNA插入将GAGAG序列与嘧啶序列分开时，从弱杂种到更稳定的杂种，RNAP回溯是有利的。 RNAP回溯对Gre因子，D-半乳糖和反义寡核苷酸敏感。天然DNA序列覆盖gal操纵子中的转录延伸区的能力揭示了以前未知的调控大肠杆菌中gal代谢的方法。它还解释了在没有生物合成反应诱导剂的情况下gal酶的合成。

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