Is hsdRMS- also dam-?

Robert Whittier rfwhittier at
Tue Nov 27 23:39:18 EST 2001

>I have problems in cleaving my vector with ClaI [. . .]


Yes, you are right that growth of the vector in a dam- strain is
necessary, and Duncan Clark is correct in pointing out that dam is
a separate locus from that coding for the restriction modification

Be happy that they are separate loci. You would not want to grow
constructs routinely in dam- strains, because these have elevated
mutation rates. In E. coli the dam methylase allows the mismatch
repair system (encoded by mutS, mutL and mutH) to discriminate
between the newly synthesized and pre-existing strands, so that
the newly synthesized (still unmethylated) strand gets repaired.


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