A plasmid vector characterized by having a promoter sequence that can be recognized by an RNA polymerase which is not inherent in a host and that controls the expression of target genes and a replication origin that increases the number of copies under the induction by foreign factors; methods of expression and isolation of target genes by using the vector; a polypeptide having the activity of an AccIII restriction enzyme; and a DNA encoding the polypeptide. The invention provides for the first time a plasmid vecto r which can introduce a foreign target gene encoding proteins which are fatal or noxious to hosts into said hosts, a method of efficiently expressing the proteins by using the vector, and also a method of permitting a restriction enzyme gene constituting a restriction-modification system to be isolated even in the absence of a modification enzyme gene, which has been difficult according to prior art.