ENZYMATIC CLEAVAGE OF A BACTERIAL CHROMOSOME AT A TRANSPOSON-INSERTED RARE SITE

The sequential use of the methylase M.Xbal (5'-TCTAG(m6)A) and the methylation-dependent endonuclease Dpnl (5'-G(m6)A down TC) results in cleavage at 5'-TCTAGA down TCTAGA. This recognition sequence was introduced into a transposon derived from the Mu bacteriophage and transposed into the genome of the bacterium Salmonella typhimurium. M.Xbal methylation was provided in vivo by a plasmid containing the M.Xbal gene and the S. typhimurium genome was cleaved to completion by Dpnl at one or more sites, depending on the number of transposon insertions. The resulting genomic fragments were resolved by pulsed-field electrophoresis. The potential use of single M.Xbal/Dpnl cleavage sites as reference positions to map rare restriction sites is discussed.