Construction of a cloning vector from a naturally occurring plasmid of Salmonella typhimurium

A naturally occurring plasmid isolated from a drug-resistant strain of Salmonella typhimurium (993) has been used to construct a plasmid vector for cloning in a wild strain of Salmonella. The strain (993) contains at least two plasmids. The smaller plasmid (9 kb) contains an ampicillin-resistant marker, while the larger one (25 kb) is cryptic. Physical mapping of the 9-kb plasmid and construction of a 3.5-kb derivative have been carried out. This plasmid has been used for cloning in a restriction+modification+strain of S. typhimurium using a conventional calcium chloride method. It exhibited better efficiency of transformation than other commonly used plasmids such as pBR322 or its derivatives and transformants were found to be stable in the absence of antibiotic selection. The vector is compatible with pBR322 and can be used to study the expression of cloned genes in minicells.