However, upon completion of their lytic cycle, they exit the cell using lysozymes (Moak & Molineux, 2000), which hydrolyze the same peptidoglycan bond as LTs do, but without the creation of anhydromuropeptides. ORFs encoding enzymes with LT active site-like domains (Blackburn & Clarke, 2001) have been identified within chromosomal or plasmid-borne operons associated with T3S and T4S systems (Koraimann, 2003). Koraimann (2003) termed these putative LTs ‘specialized LTs’ to indicate that they have a unique biological function Osimertinib chemical structure not associated with basic peptidoglycan metabolism.

The peptidoglycan-lytic activity of putative specialized LTs has often been demonstrated with zymograms on peptidoglycan-containing gels. However, proteins that bind but do not hydrolyze peptidoglycan can still produce zones of clearing on a zymogram by sequestering peptidoglycan away from the stain; for this reason, zymograms intended to demonstrate lytic activity should be interpreted with caution (Dijkstra & Keck, 1996b; Kohler et al., 2007). Work by Zahrl et al. (2005) and Kohler et al. (2007) demonstrated cleavage specificity against the MurNAc-GlcNAc linkage for a number of specialized LTs involved in T3S (IpgF, Shigella Raf inhibitor flexneri; IagB, Salmonella enterica) and T4S (VirB1, Agrobacterium

tumefaciens, Brucella suis; TrbN, Pseudomonas sp.; P19, E. coli plasmid R1; HP0523, Helicobacter pylori; AtlA, Neisseria gonorrhoeae). AtlA, one of two N. gonorrhoeae LTs involved in T4S (Kohler et al., 2005, 2007), was also shown to produce 1,6-anhydromuropeptides, the definitive

sign of an LT-catalyzed reaction. Degradation by AtlA does not appear to contribute to the overall pools of peptidoglycan monomer that N. gonorrhoeae releases to the extracellular environment, suggesting that its activity is reserved for the creation of localized gaps to permit T4S system assembly (Kohler et al., 2007). Although specialized LTs degrade peptidoglycan, their activities are typically nonessential; loss of the putative LT in most cases decreases, but does not abrogate, secretion of effectors and thus virulence. The observed decreases are often due to a reduction in surface components including flagellin or needle filaments, pilin (Viollier & Shapiro, 6-phosphogluconolactonase 2003; Hoppner et al., 2004; Yu et al., 2010), and in some cases, structural components from the inner or outer membranes (Baron et al., 1997; Viollier & Shapiro, 2003). As most bacteria encode a number of different LTs, it is likely that assembly of T3S and T4S complexes can continue, albeit less efficiently, by taking advantage of temporary breaks in the sacculus that are created during normal peptidoglycan metabolism. While most studies have examined the involvement of specialized LTs in macromolecular complex assembly, other peptidoglycan-degrading activities may also be involved in this process. In fact, three different enzymatic mechanisms of peptidoglycan cleavage have been associated with flagellar assembly.