INFLUENCE OF DISRUPTION OF THE RECA GENE ON GENETIC INSTABILITY AND GENOME REARRANGEMENT IN STREPTOMYCES-LIVIDANS

Streptomyces lividans TK23 gives rise to chloramphenicol-sensitive (Cm l(s)) mutants at a frequency of about 0.5%. This is due to the frequen t occurrence of very large chromosomal deletions removing the correspo nding 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 en hancement of the deletion frequency was responsible for this mutator p henotype. The amplifiable locus AUD1 has a duplicated structure in som e S. lividans strains and is frequently highly amplified in some mutan ts generated by genetic instability. The chromosomal AUD1 is not ampli fied in strain TK23 because of the lack of one duplication. Neverthele ss, AUD1-derived amplifiable units presenting the typical duplicated o rganization amplified very well in TK23 when carried on a plasmid. No amplification of these units was observed in the recA mutant. The abil ity to amplify was restored when the wild-type recA gene was introduce d into the plasmid carrying the amplifiable unit. These results sugges t that the RecA protein plays a role in reducing the level of genetic instability and chromosomal deletions and show that the recA gene is n ecessary to achieve high-copy-number amplification of AUD1.