Previous studies have shown that M13 single-strand synthesis is inhibited at nonpermissive temperature in Escherichia coli polAexl, a temperature-sensitive mutant defective in the 5' -. 3' exonuclease activity of polymerase I (T.-C. Chen and D. S. Ray, J. Mol. Biol. 106:589-604, 1976). Under these conditions the formation of covalently closed replicative form (RF) molecules is greatly reduced, and miniature forms of RF accumulate. We show here that the accumulation of mini-RFs is the

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... of a differential inhibition of the replication of unitlength phage and preexisting miniphage rather than a de novo production of miniphage. Mini-RFs do not accumulate even after as many as nine cycles of