ARTIFICIAL CIRCULARIZATION OF THE CHROMOSOME WITH CONCOMITANT DELETION OF ITS TERMINAL INVERTED REPEATS ENHANCES GENETIC INSTABILITY AND GENOME REARRANGEMENT IN STREPTOMYCES-LIVIDANS

The unstable linear chromosome of Streptomyces lividans was circulariz ed by homologous recombination and its terminal inverted repeats delet ed. Strains with circularized chromosomes showed no obvious phenotypic disadvantages compared to the wild type. However, they segregated abo ut 20 times more chloramphenicol-sensitive mutants than the wild type (24.3% vs. 1.4%), due to a higher incidence of large deletions. In add ition, in all circularized chromosomes amplification of 30-60 kb fragm ents was observed at the new chromosomal junction, to levels of approx imately 10 copies per chromosome. Arginine auxotrophs that arose spont aneously among the progeny of strains with a circularized chromosome s howed high-copy-number amplification of the DNA element AUD1, as also seen in mutants of the wild type. These observations demonstrate that the circular form of the Streptomyces chromosome is more unstable than the linear one.