Discussion
We have developed a highly efficient intergeneric conjugation
system to transfer plasmids from E. coli to S. rimosus.
This method has proved to be easy, convenient and
less time-consuming compared to the classical PEG-mediated
protoplast transformation method and required no
expensive equipment such as that required for electroporation.
Conditions were then optimized for efficient intergeneric
conjugal transfer into S. rimosus. Optimal results
were obtained when S. rimosus spores were activated by
heat treatment at 40C for 10 min before mating with
E. coli; this had little effect on spore viability and gave
high conjugation efficiencies. This condition is similar to
that used with S. lavendulae (Kitani et al. 2000). Interestingly,
the conjugation efficiencies from nonheated spores
of S. rimosus were relatively high compared to those
reported for the heat-sensitive spores of S. virginiae
(2 · 10)3
, Voeykova et al. 1998) and S. ambofaciens
(1Æ4 · 10)2
, Kim et al. 2008) and was much higher than
untreated spores of S. albulus (9 · 10)8
, Hamano et al.
2005). In S. natalensis, the use of nonheated spores gave
no transconjugants (Enr´iquez et al. 2006). Thus, unlike
some other streptomycetes, effective conjugal transfer into
S. rimosus could be obtained either with or without heat
treatment of spores.