Stenotrophomonas maltophilia KH has two acid beta-lactamases with isoelectric points (pls) of 4.6 and 5.4, and several basic beta-lactamases (pls > 7.0) that produce a ladder-shaped pattern by IEF. An isogenic L2 mutant, KHL2(xylE), was constructed by gene replacement. From IEF and native PAGE zymograms of strains KH and KHL2(xylE), it was demonstrated that the basic beta-lactamases and the acid beta-lactamase with pI 5.4 are encoded by the same L2 gene and that the active types of these L2 charge variants were dependent on the buffer pH. The beta-lactamase activities of these L2 charge variants in phosphate buffer at pH 7.0 and 8.0 were 1075 29 and 1114 +/- 81 U mg(-1), respectively. These results indicate that L2 charge variants give S. maltophilia a better chance of adapting and surviving in response to changes in the environment.
