Plasmid ColE1 incompatibility determined by interaction of RNA I with primer transcript.

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Abstract
Mutants of plasmid pNT7 that can coexist with plasmid pMB9 in growing bacteria (Escherichia coli) were isolated. These mutants show altered incompatibility properties and increased copy numbers. Each mutant has a single base change at or near the center of 1 of the 3 palindromes in the region that specifies (2) RNA species: a larger one (primer transcripts) that provides a primer for DNA replication and a smaller one (RNA I) that is the incompatibility-specific inhibitor of primer formation. In vitro transcription studies show that the single base changes affect both the ability of RNA I to inhibit primer formation and the sensitivity of primer formation to inhibition by RNA I. RNA I hybridizes to the primer transcript, and the rate of hybridization is reduced by the single base changes. Based on analyses of inhibition of in vitro primer formation by RNA I and of in vivo properties of the mutant plasmids, it was concluded that incompatibility between 2 plasmids can be attributed to inhibition of primer formation on 1 of the plasmids by the RNA I of the other. Inhibition of primer formation by RNA I appears to be the mechanism that determines the copy number of pNT7 and its derivatives.