The Pneumococcal Cell Envelope Stress-Sensing System LiaFSR Is Activated by Murein Hydrolases and Lipid II-Interacting Antibiotics

Abstract

In the Firmicutes, two-component regulatory systems of the LiaSR type sense and orchestrate the response to various agents that perturb cell envelope functions, in particular lipid II cycle inhibitors. In the current study, we found that the corresponding system in Streptococcus pneumoniae displays similar properties but, in addition, responds to cell envelope stress elicited by murein hydrolases. During competence for genetic transformation, pneumococci attack and lyse noncompetent siblings present in the same environment. This phenomenon, termed fratricide, increases the efficiency of horizontal gene transfer in vitro and is believed to stimulate gene exchange also under natural conditions. Lysis of noncompetent target cells is mediated by the putative murein hydrolase CbpD, the key effector of the fratricide mechanism, and the autolysins LytA and LytC. To avoid succumbing to their own lysins, competent attacker cells must possess a protective mechanism rendering them immune. The most important component of this mechanism is ComM, an integral membrane protein of unknown function that is expressed only in competent cells. Here, we show that a second layer of self-protection is provided by the pneumococcal LiaFSR system, which senses the damage inflicted to the cell wall by CbpD, LytA, and LytC. Two members of the LiaFSR regulon, spr0810 and PcpC (spr0351), were shown to contribute to the LiaFSR-coordinated protection against fratricide-induced self-lysis.

In-Depth Profiling of the LiaR Response of Bacillus subtilis

“…Homologs of the liaIH operon are absent in all Firmicutes cocci, indicating a functional diversification between the two groups of LiaFSRlike systems. These functional differences are also supported by the LiaR binding site of the Bacillus/Listeria group, which differs significantly from the consensus sequence determined for the Firmicutes cocci (16).…”

“…2A and D). While the four core residues at the ends of the two repeats are conserved (boldface), only two more residues fit to the position weight matrix (capital letters) (16), and no additional complementary bases could be detected. To exactly map the LiaR binding site upstream of ydhE, two additional fragments (Ϫ74 and Ϫ71) were cloned and corresponding reporter strains (TMB676 and TMB769) constructed, and ␤-galactosidase activities were measured in the liaF mutant background.…”

“…Studies of LiaFSR-like systems in four different Firmicutes bacteria allowed the identification of a conserved inverted repeat element as the binding sequence for LiaR-like response regulators (16,29,43,55). While there is some variation between binding sites from Bacillus/Listeria species (group I) and Firmicutes cocci (group II), a minimal and almost invariant core 6-4-6 motif, TNNNNC----GNNNNA, can be extracted, with normally at least two additional residues having complementary partners in each side of the repeat (16). While this rule certainly applies for the binding site of the liaI promoter, the inverted repeats identified upstream of yhcY and ydhE are poorly conserved (Fig.…”

“…More recently, a number of detailed biochemical studies have addressed both the phosphotransfer between the sensor kinase VraS and the response regulator VraR and the phosphorylation-dependent DNA-binding specificity of VraR (5,6,15,39). Other Lialike systems that have been genetically and physiologically investigated include CesSR of Lactococcus lactis and the recently described LiaFSR system of Streptococcus pneumoniae (16,43).…”

Stress Physiology of Lactic Acid Bacteria

“…Disruption of this system in S. mutans led to augmented sensitivity to antibiotics interfering with lipid II cycling (442). A system homologous to LiaFSR has also been characterized for S. pneumoniae, in which it was shown to control the expression of at least 19 genes involved in protecting cells against murein hydrolases and lipid II-interacting antibiotics (449). In S. agalactiae, this system controls not only cell wall stress-related responses but also pilus expression and genes involved in the defense against host antimicrobial systems (440).…”