Summary: A detailed physical map of the region of the IncFI plasmid ColV2-K94 containing the Rep1 replicon, a Tn transposon, and an inverted repeat structure (X) with unknown properties was prepared by cloning restriction fragments into pBR325. Inserts carrying the 1·2 kb repeated sequence of X, but not the IS sequence of Tn, had a destabilizing effect on pBR325 and pBR322 plasmid maintenance. One of these derivatives, pWS139, was studied further and was shown to have elevated levels of multimeric DNA forms; this resulted in decreased copy number and plasmid instability, as multimerization reduces the effective number of randomly segregating plasmids per cell. A ColV2-K94 miniplasmid, which has a copy number much lower than that of ColE1-derived vectors, was also less stably inherited if it contained the X structure. This destabilizing effect of the X repeat sequence was dependent on the RecA function, but not the RecB or the RecC functions of the host. These results suggest that the inverted repeat sequence of the X structure serves as a ‘hot-spot’ for generalized recombination. Thus, when present in , this sequence can generate plasmid instability because plasmid molecules are readily converted into multimeric forms through enhanced recombination at this site.


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