Characterization of the role of LtgB, a putative lytic transglycosylase in Neisseria gonorrhoeae.

Neisseria gonorrhoeae releases monomeric peptidoglycan (PG) fragments during growth. These PG fragments affect pathogenesis-related phenotypes including induction of inflammatory cytokines and killing of ciliated fallopian tube cells. Although the biological activities of these molecules have been established in multiple systems, the genes and gene products responsible for their production in N. gonorrhoeae have not been determined. The authors previously identified genes for three lytic transglycosylase homologues (ltgA, ltgB and ltgC) in the N. gonorrhoeae genome sequence. Mutation of ltgA was found to affect PG fragment release, and mutation of ltgC affected cell separation. In this study the effects of complete deletion or point mutations in ltgB were characterized. Point mutations were introduced by a combination of insertion-duplication mutagenesis and positive and negative selection, thereby generating selectable marker-less mutations. The ltgB deletion mutant had normal growth characteristics and was not affected in PG fragment release. When expressed in Escherichia coli, gonococcal LtgB was able to substitute for lambda endolysin to cause cell lysis. Mutation of the predicted catalytic-site glutamic acid residue did not decrease lysis in this system. However, mutation of a nearby glutamic acid residue eliminated lysis activity.