Long palindromes formed in Streptomyces by nonrecombinational intra-strandannealing

Long inverted repeats (palindromes) are ubiquitous among prokaryotic and eu karyotic genomes. Earlier work has implicated both DNA breaks and short inv erted repeats (IRs) in the formation of long palindromes in yeast and Tetra hymena by a proposed mechanism of intramolecular recombination. Here we rep ort that long-palindromic linear plasmids are formed in Streptomyces follow ing double strand DNA breakage by a nonrecombinational intra-strand anneali ng process that also involves IRs. By modification of palindrome-generating linear plasmids and development of a novel procedure that enables the sequ encing of palindrome junctions, we show that long-palindrome formation occu rs by unimolecular intra-strand annealing of IRs followed by 3' extension o f the resulting DNA fold-back. The consequent hairpin structures serve as t emplates for synthesis of duplex linear plasmids containing long palindrome s. We suggest that this model for long-palindrome formation in Streptomyces may represent a generally applicable mechanism for generating DNA palindro mes.