SOS induction by P1 Km miniplasmids

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Abstract
A set of P1 miniplasmids were constructed in Escherichia coli. The smallest of these that could function as an independent replicon contained the right side of EcoRI-5 plus all of EcoRI-8. Those miniplasmids that lack EcoRI-6 induce the SOS pathway of the cell as shown by increased expression of the recA operon, excision of the cryptic genetic element e14, spontaneous induction of .lambda. and dependence of e14 excision on recA+ function. This induction was contingent upon the replication of the P1 Km miniplasmids from their P1 origin and was apparently caused by an aberrant initiation of DNA replication. When P1 EcoRI-6 was present in cis or trans with a P1 Km miniplasmid, neither e14 nor .lambda. was excised, but the expression of the recA operon was still induced. P1 EcoRI fragments 5 and 8 are apparently insufficient for normal replication and the P1 Km miniplasmids induced SOS functions. A product of EcoRI-6 may partially restore normal replication.