The terminal structures of linear plasmids from Rhodococcus opacus.

The telomers of several linear plasmids of Rhodococcus opacus (formerly Nocardia opaca) were studied. The plasmids pHG201, pHG204 and pHG205 carry proteins bound to their ends, as shown by gel retardation experiments. A sequence hybridizing with the terminal sequence of pHG207, a recombinant linear plasmid consisting of the left part of pHG204 and the right part of pHG205, which was analysed in a previous study by the authors, could be detected in all linear plasmids of the wild-type R. opacus strains MR11 and MR22. However, only pHG204 and pHG206 carry terminal inverted repeats (TIRs) like pHG207. Cloning and sequencing of the terminal fragment of pHG204 revealed a nearly perfect TIR of 1016 bp. In contrast, the termini of pHG201 and pHG205 share little homology. Sequence analysis of the two end fragments of pHG201 revealed a similarity of only 65% within the terminal 34/32 bp and a perfect TIR of only 3 bp. The results support the assumption that long TIRs are not absolutely necessary for replication and maintenance of linear plasmids.