Volff J N, Altenbuchner J
Institut für Industrielle Genetik, Universität Stuttgart, Germany.
J Bacteriol. 1997 Apr;179(7):2440-5. doi: 10.1128/jb.179.7.2440-2445.1997.
Streptomyces lividans TK23 gives rise to chloramphenicol-sensitive (Cml(s)) mutants at a frequency of about 0.5%. This is due to the frequent occurrence of very large chromosomal deletions removing the corresponding chloramphenicol resistance gene. A mutant in which the recA gene has been disrupted (S. lividans FrecD3 [G. Muth, D. Frese, A. Kleber, and W. Wohlleben, personal communication]) segregated about 70 times more chloramphenicol-sensitive mutants than the parental strain. An enhancement of the deletion frequency was responsible for this mutator phenotype. The amplifiable locus AUD1 has a duplicated structure in some S. lividans strains and is frequently highly amplified in some mutants generated by genetic instability. The chromosomal AUD1 is not amplified in strain TK23 because of the lack of one duplication. Nevertheless, AUD1-derived amplifiable units presenting the typical duplicated organization amplified very well in TK23 when carried on a plasmid. No amplification of these units was observed in the recA mutant. The ability to amplify was restored when the wild-type recA gene was introduced into the plasmid carrying the amplifiable unit. These results suggest that the RecA protein plays a role in reducing the level of genetic instability and chromosomal deletions and show that the recA gene is necessary to achieve high-copy-number amplification of AUD1.
淡紫灰链霉菌TK23产生氯霉素敏感（Cml(s)）突变体的频率约为0.5%。这是由于经常发生非常大的染色体缺失，从而去除了相应的氯霉素抗性基因。recA基因被破坏的一个突变体（淡紫灰链霉菌FrecD3 [G. 穆特、D. 弗雷泽、A. 克莱伯和W. 沃尔勒本，个人交流]）分离出的氯霉素敏感突变体比亲本菌株多约70倍。缺失频率的增加导致了这种突变体表型。可扩增位点AUD1在一些淡紫灰链霉菌菌株中具有重复结构，并且在由遗传不稳定性产生的一些突变体中经常高度扩增。由于缺少一个重复序列，染色体上的AUD1在菌株TK23中未被扩增。然而，呈现典型重复组织的源自AUD1的可扩增单元在携带于质粒上时在TK23中扩增得非常好。在recA突变体中未观察到这些单元的扩增。当将野生型recA基因导入携带可扩增单元的质粒时，扩增能力得以恢复。这些结果表明RecA蛋白在降低遗传不稳定性和染色体缺失水平方面发挥作用，并表明recA基因是实现AUD1高拷贝数扩增所必需的。
