Nucleotide sequence of a 7-kb fragment of pACM1 encoding an IncM DNA primase and other putative proteins associated...

A 7-kb fragment of pACM1 (fragment 90¿91) containing one or more kor (kill-override) loci was sequenced, and 28 open reading frames (ORFs; >/=50 codons) were identified. The nucleotide sequence has no significant homologs in the GenBank database except for a 1.3-kb region 98.6% identical to the iml (insensitivity to phage PhiM-mediated lysis) determinant fragment of IncM plasmid R446. Deduced amino acid sequences for several ORFs are homologous to those of known proteins, including the Sog DNA primases of IncI1 plasmids R64 and ColIb-P9 and the TraL, TraM, and TraN products of ColIb-P9. Two protein products of the putative primase ORF (ORF 1, 1100 amino acids) were detected by SDS-PAGE. The 158- and 107-kDa proteins were designated PriL and PriS, respectively. PriS is apparently produced by an in-frame reinitiation of the ORF 1 transcript at a second start codon located between a Sau96I site and a PstI site. The motif EGYATA, conserved among primases and associated with primase function, occurs in the first one-third of the deduced amino acid sequence of PriL and is not included in PriS. Partial suppression of the temperature-sensitive dnaG3 mutation in BW86 was demonstrated by recombinants that overexpressed both PriL and PriS, but not by constructs overexpressing only PriS. Therefore, primase function can be assigned to PriL. Fragment 90/91 represents a portion of the IncM tra region, which has not previously been examined in detail.