A runaway-replication plasmid pSY343 contains two ssi signals

Taking advantage of the plaque morphology method, we detected two single-stranded initiation (ssi) signals in the plasmid pSY343; one was in the 170-nucleotide (nt) EcoRV-ThaI segment (170P), and the other was in the 93-nt DraI-FnuDII segment (93F), which were designated as ssiA and ssiB, respectively. We cloned the two ssi signals in the filamentous phage vectors M13 delta lac184 and flR199. A conserved 7-nt consensus sequence involved in the n' recognition site for priming DNA initiation on single-stranded (ss) DNA templates (A. Van der Ende, R. Teerstra, H. Van der Avoort, and P.J. Weisbeek, 1983, Nucleic Acids Res. 11, 4957-4975) was found, three copies in 170P and one in 93F. These two ssi signals contain possible stem and loop structures. The 170P overlapped partly with the origin (ori) region of pSY343 and the 93F was away from the ori region. Growth of chimera phages such as M13 delta lac184/ssiA and M13 delta lac184/ssiB was 38- and 71-fold greater, respectively, than that of M13 delta lac184, 8 h after phage infection. The conversion efficiency in vivo of ss to replicative form (RF) DNA of these chimera phages carrying ssiA and ssiB was 1.9- and 2.2-fold greater, respectively, than that of M13 delta lac184, 50 min after infection.