EXPRESSION OF A DNA METHYLATION (DAM) GENE IN ESCHERICHIA-COLI K-12

Plasmid pMQ3, carrying the dam gene of Escherichia coli on a 6.1 Kb fragment, shows a tenfold increase in relative DNA adenine methylase activity, while plasmid p dam118, with a 1.14 Kb dam insert, shows only a twofold increase, although both plasmids were derived from plasmid pLC13-42. Since a copy number effect did not seem to be the cause of this difference, we have subcloned pMQ3 in order to determine whether the additional chromosomal DNA present in this plasmid is responsible for the enhancement of methylase activity. We show that the 346 base pairs upstream of dam contain sequences necessary for expression. DNA sequence analysis has revealed that in p dam118 only the 118 bases 5-prime to the dam gene are present in other constructs and that the additional upstream material is pBR322 DNA. This shows that p dam118 carries a DNA duplication. © 1990 Springer-Verlag New York Inc.