Reversion and immobilization of phage Mu d1 cts (Ap^rlac) insertion mutations in Salmonella typhimurium

Phage Mu d1 cts (Ap^rlac), or Mu d1, insertion mutations may be accompanied by adjacent deletion formation which can complicate use of lac fusions generated with this phage for gene regulatory studies. As for phage Mu insertion mutations, phage Mu d1 insertions fail to revert at significant frequency (whether or not accompanied by an adjacent deletion). We describe isolation of revertible (X mutant) derivatives of phage Mu d1 in Salmonella typhimurium. The X mutant derivatives allow use of reversion as a simple test to determine whether a Mu d1 insertion has occurred precisely without an adjacent deletion that may have fused the lac genes to a promoter outside of the gene of interest. In addition, a simple method for stabilizing Mu d1 generated lac fusions against subsequent transposition is described.