A predicted T4 secretion system and conserved DNA-repeats identified in a subset of related Arthrobacter plasmids

BLAST analysis of pAO1 ORFs of Arthrobacter nicotinovorans revealed 12 ORFs, including the ORF of a transcriptional regulator, predicted to encode the components of a T4-secretion system involved in bacterial conjugation. These ORFs were conserved and showed synteny among 14 Arthrobacter plasmids. A DNA repeat of about 370 nucleotides was found to be present 5' to the pAO1 ORFs of DUF4192-, DprA- and ParB-like proteins. Similar repeats were present in identical positions on 12 additional Arthrobacter plasmids. The DNA repeats on a particular plasmid are highly identical duplications. The DNA repeats contain alternating GC and AT reach sequences, potential protein DNA-binding sites and purine reach stretches. The sequences end with 5′ATG.AAC3′ which results in the amino terminal sequence methionine (M) and asparagine (N) for all predicted DprA, DUF4192 and ParB proteins. The presences of conserved ORFs of a T4-secretion system and of similar DNA repeats suggest that these Arthrobacter plasmids are related and evolved from a common ancestor. The functional significance of the DNA repeats in a coordinated common mechanism of regulation of expression of the dprA-(involved in natural competence), parB- (involved in plasmid partitioning) and duf4192- (unknown function in plasmid life cycle) genes remains to be established.