Two classes of recA mutations have been constructed for use in Neisseria gonorrhoeae: three insertionally inactivated ('knockout') mutations and three LacI-regulatable constructs that can be shifted between Rec- and Rec+ by the removal or addition of IPTG. The effects of regulating recA expression on the processes of DNA transformation, DNA repair and pilin-phase variation are described. These regulatable cassettes can also be used to control the expression of any chromosomal gene.