Cheng K C, Smith G R
Fred Hutchinson Cancer Research Center, Seattle, Washington 98104.
Genetics. 1989 Sep;123(1):5-17. doi: 10.1093/genetics/123.1.5.
The recombination hotspot Chi, 5' G-C-T-G-G-T-G-G 3', stimulates the RecBCD recombination pathway of Escherichia coli. We have determined, with precision greater than previously reported, the distribution of Chi-stimulated exchanges around a Chi site in phage lambda. Crosses of lambda phages with single base-pair mutations surrounding a Chi site were conducted in and analyzed on mismatch correction-impaired hosts to preserve heteroduplex mismatches for analysis. Among phages recombinant for flanking markers, Chi stimulated exchanges most intensely in the intervals immediately adjacent to the Chi site, both to its right and to its left. Stimulation fell off abruptly to the right but gradually to the left (with respect to the orientation of the Chi sequence written above). We have also determined that Chi stimulated the formation of heteroduplex DNA, which frequently had one endpoint to the right of Chi and the other endpoint to the left. These data support a model of Chi-stimulated recombination in which RecBCD enzyme cuts DNA immediately to the right of Chi and unwinds DNA to the left of Chi; segments of unwound single-stranded DNA are sometimes, but not always, degraded before synapsis with homologous DNA.
重组热点序列Chi（5' G-C-T-G-G-T-G-G 3'）可刺激大肠杆菌的RecBCD重组途径。我们已以前所未有的精度确定了噬菌体λ中Chi位点周围受Chi刺激的交换分布情况。在错配修复功能受损的宿主中进行并分析了围绕Chi位点带有单碱基对突变的λ噬菌体的杂交实验，以保留异源双链错配用于分析。在侧翼标记发生重组的噬菌体中，Chi在紧邻Chi位点的区间（位点右侧和左侧）刺激交换的强度最大。刺激作用在右侧急剧下降，而在左侧逐渐下降（相对于上述Chi序列的方向）。我们还确定Chi刺激了异源双链DNA的形成，其常常一个端点在Chi右侧，另一个端点在Chi左侧。这些数据支持了一种Chi刺激重组的模型，其中RecBCD酶在Chi右侧紧邻处切割DNA，并在Chi左侧解旋DNA；解旋的单链DNA片段在与同源DNA联会之前有时会被降解，但并非总是如此。
