Actinomycetes DNAs were digested with restriction enzymes to study the
presence of methylated bases. Analysis showed that the enterobacteria
l Dam and Dcm systems are absent. Methylation at the internal cytosine
in CCGG sequences, typical of eukaryotes, was also absent. We also te
sted 18 restriction endonucleases recognizing six base pair sequences
(all of which were inhibited by methylation). Results showed a higher
number of restriction sites for enzymes recognizing CG-rich sequences
(CG endonucleases) than for enzymes recognizing AT-rich sequences (AT
endonucleases). Restriction patterns with CG endonucleases were quite
uniform, with the remarkable exception of XhoI, which yielded a small
number of DNA bands. The study performed with AT endonucleases allowed
differentiation of three groups of enzymes based on different degrees
of chromosomal sensitivity. One group (BglII and BglII) produced rest
riction patterns with more abundant restriction sites than expected, a
second group (ClaI, EcoRI, and EcoRV) yielded the predicted number of
DNA fragments, and the third group (HpaI, HindIII, XbaI, and DraI) pr
oduced an unexpectedly low number of fragments. Some individual cases
of resistance to particular enzymes could be explained by the presence
of restriction-modification systems with the same specificity.