New replication mutant pNH602 and its relationship to plasmid pAS3, another deletion derivative of plasmid R6K

A stable deletion derivative pNH602 was obtained when the recently described higher-copy-number point mutant pNH601 of plasmid R6K was introduced to a minicells-producing strain of Escherichia coli. The size of plasmid pNH602 is 18.8 Mg/mol as determined by electron microscopy. The 7.2 Mg/mol fragment of R6K genome missing in pNH602 carries the Smr-determinant and the region finO and, according to the results of restriction analysis, it includes one EcoRI site. With its radioisotopically determined 33 copies of pNH602 per E. coli K-12 chromosome (npc), representing a 23% increase of the point mutant pNH601 and 150% enhancement of R6K npc, plasmid pNH602 differs from another closely related R6K deletion derivative pAS3 of the same size which exhibits only 20 npc. Both pNH602 and pAS3 plasmids are conjugative.