FIG. 4.

(A) Identification of a flaK deletion mutant by a PCR screen of individual transformants. The PCR primers were designed to amplify across the flaK gene region and give a 1,000-bp product if the flaK gene is intact and a 300-bp product if the flaK gene is deleted. Of the nine transformants tested in this run, only number 5 was a mutant. (B) Confirmation of flaK deletion by Southern blotting. Genomic DNA was digested with RsaI and hybridized with a probe specific for the flaK surrounding region. Wild-type (lane 2) and mutant DNA (lane 3) yielded bands that matched the predicted sizes of 2.1 kb and 1.4 kb, respectively. Lane 1 is λ-HindIII-digested DNA marker (23, 9, 6.5, 4.3, 2.3, 2, and 0.5 kb; the last marker band is not seen on this gel).