Replication-involved genes of pAL1, the linear plasmid of Arthrobacter nitroguajacolicus Rü61a – Phylogenetic and transcriptional analysis

The 113-kb pAL1 is the only Arthrobacter linear plasmid known; it has terminal inverted repeats and 5′ covalently attached terminal proteins (TPs). The latter and a telomere-associated protein (Tap) are encoded by plasmid ORFs 102 and 101, respectively. As for Streptomyces linear replicons, in which both above proteins are instrumental in telomere patching, they are involved in pAL1 replication as well. However, the alignment of actinobacterial Taps and TPs revealed that pAL1 and the linear elements from Rhodococci comprise a discrete phylogenetic group, clearly delineated from the streptomycetes linear plasmids. In line with such findings is the same genetic arrangement of ORF 101 and 102 counterparts in the rhodococcal elements. Furthermore, the adjacent gene (ORF100) has matches in the rhodococcal plasmids as well. In linear elements of Streptomyces there is no ORF100 homolog. Two alternative annotations are possible for ORF100 gene products. As RT-PCR revealed cotranscription of ORFs 100–102, the ORF100 gene product is presumably involved in replicative processes. Taken also into consideration the likely absence of an internal replication origin (other than in Streptomyces linear elements), we assume a distinct replication/telomere patching mechanism for pAL1 type replicons.