The integrative element pSAM2 from Streptomyces: kinetics and mode of conjugal transfer

pSAM2 is an 11 kb integrative element from Streptomyces ambofaciens that is capable of conjugal transfer. A system based on differential DNA modification by SalI methyltransferase was used to localize pSAM2 in the donor or recipient strain, and thus to determine the various steps associated with transfer. Initiation (i.e. excision and replication of pSAM2 in the donor) occurs a few hours after mating with a recipient strain. pSAM2 replicates in the recipient strain, spreads within the mycelium and then integrates into the chromosome. Transfer generally involves single-stranded DNA. In Streptomyces, only a few genes, such as traSA for pSAM2, are required for conjugal transfer. Using the differential sensitivity to the SalI restriction–modification system of transfers involving single- and double-stranded DNA, we found that pSAM2 was probably transferred to the recipient as double-stranded DNA. This provides the first experimental evidence for the transfer of double-stranded DNA during bacterial conjugation. Thus, TraSA, involved in pSAM2 transfer, and SpoIIIE, which is involved in chromosome partitioning in Bacillus subtilis, display similarities in both sequence and function: both seem to transport double-stranded DNA actively, either from donor to recipient or from mother cell to prespore.