A new procedure for the targeted inactivation of essential bacterial genes.

A new method is described for the exchange of a plasmid encoded mutant gene with its chromosomal counterpart. The method is based on positive selection and is applicable for the exchange of essential genes. The main features of the method are: (1) cloning of an antibiotic resistance marker (without its promoter) downstream of the cloned target gene, thus forming an artificial operon; (2) inactivating the target gene (and consequently also the antibiotic resistance gene) by inserting a strong transcriptional termination signal into it; and (3) selection for double recombinants by means of the antibiotic resistance gene.