The
highly conserved WalK/WalR (also known as YycG/YycF)
two-component system is specific to low-G+C
gram-positive bacteria. While this system is essential for cell viability, both the nature of its
regulon and its physiological role have remained mostly uncharacterized. We observed that, unexpectedly,
Staphylococcus aureus cell death induced by WalKR depletion was not followed by
lysis. We show that WalKR positively controls
autolytic activity, in particular that of the two major
S. aureus autolysins, AtlA and LytM. By using our previously characterized consensus WalR
binding site and carefully reexamining the genome annotations, we identified nine genes potentially belonging to the WalKR
regulon that appeared to be involved in
S. aureus cell wall degradation. Expression of all of these genes was positively controlled by WalKR levels in the cell, leading to high resistance to Triton X-100-induced
lysis when the cells were starved for WalKR. Cells lacking WalKR were also more resistant to lysostaphin-induced
lysis, suggesting modifications in
cell wall structure. Indeed, lowered levels of WalKR
led to a significant decrease in
peptidoglycan biosynthesis and turnover and to
cell wall modifications, which included increased
peptidoglycan cross-linking and
glycan chain length. We also demonstrated a direct relationship between WalKR levels and the ability to form
biofilms. This is the first example in
S. aureus of a regulatory system positively controlling
autolysin synthesis and
biofilm formation. Taken together, our results now define this
signal transduction pathway as a master regulatory system for
cell wall metabolism, which we have accordingly renamed WalK/WalR to reflect its true function.
