Enhanced instability of incfii basic replicon by the polA mutation

Summary IncFII plasmids consisting of a basic replicon and of an additional fragment(s) unrelated to plasmid maintenance were all less stable in polAl than in its wild type. Reversion to UV-resistance recovered stability and vice versa . UV irradiation promoted instability in polAl cells. Larger plasmids showed a greater instability and a fewer number of copies in a same host. Surprisingly, polAl cells with Tn 3 on the plasmid showed a higher ampicillin resistance than the wild type, apparently suggesting that the polAl mutation increases the copy number. The size-dependency of instability was less marked in polAl than in its parent. Comparison of the generation times has suggested adetrimental effect exerted by a basic replicon in polAl hosts.