Competence in Streptococcus pneumoniae Is Regulated by the Rate of Ribosomal Decoding Errors

Abstract

Competence for genetic transformation in Streptococcus pneumoniae develops in response to accumulation of a secreted peptide pheromone and was one of the initial examples of bacterial quorum sensing. Activation of this signaling system induces not only expression of the proteins required for transformation but also the production of cellular chaperones and proteases. We have shown here that activity of this pathway is sensitively responsive to changes in the accuracy of protein synthesis that are triggered by either mutations in ribosomal proteins or exposure to antibiotics. Increasing the error rate during ribosomal decoding promoted competence, while reducing the error rate below the baseline level repressed the development of both spontaneous and antibiotic-induced competence. This pattern of regulation was promoted by the bacterial HtrA serine protease. Analysis of strains with the htrA (S234A) catalytic site mutation showed that the proteolytic activity of HtrA selectively repressed competence when translational fidelity was high but not when accuracy was low. These findings redefine the pneumococcal competence pathway as a response to errors during protein synthesis. This response has the capacity to address the immediate challenge of misfolded proteins through production of chaperones and proteases and may also be able to address, through genetic exchange, upstream coding errors that cause intrinsic protein folding defects. The competence pathway may thereby represent a strategy for dealing with lesions that impair proper protein coding and for maintaining the coding integrity of the genome.

The HtrA Protease from Streptococcus pneumoniae Digests Both Denatured Proteins and the Competence-stimulating Peptide

“…In addition to its sensitivity to bacterial density, we have recently shown that pneumococcal competence is triggered when the frequency of biosynthetic errors during protein translation increases (8). Such responsiveness to the accuracy of protein synthesis supports the recent suggestion that competence in S. pneumoniae serves as a stress response (9) and is consistent with the observation that the competence regulon includes genes for cellular proteases and chaperones in addition to those genes responsible for transformation (10).…”

“…Such responsiveness to the accuracy of protein synthesis supports the recent suggestion that competence in S. pneumoniae serves as a stress response (9) and is consistent with the observation that the competence regulon includes genes for cellular proteases and chaperones in addition to those genes responsible for transformation (10). Although the underlying mechanism has not been defined, this error-dependent pattern of regulation is generated in part through the activity of the bacterial HtrA protease, which acts selectively to repress the development of competence when the frequency of translational errors is low (8).…”

“…A similar process was used previously in constructing strains KSP122 (htrA S234A) and KSP148 (htrA ϩ ) in the same background (8). Competence Assays-A luciferase reporter consisting of an ssbBЈ-luc transcriptional fusion was used to measure activation of the competence system as described previously (8). Expression of ssbB is specifically induced during competence, and this reporter has been demonstrated to reflect competence for transformation (31,36).…”

“…A product encoding the in-frame deletion was constructed by overlap extension PCR using the primers 5Ј-TTTTGTTTAGTGATTGGGGTAAG-3Ј and 5Ј-CTTCGAC-AATCTTGCCCTGTGGACTAGGTGCCATAAAAAGAGTC-3Ј for the 5Ј region and primers 5Ј-GACTCTTTTTATGGCACC-TAGTCCACAGGGCAAGATTGTCGAAG-3Ј and 5Ј-TAGC-GAACAGAATCACCGAC-3Ј for the 3Ј region. A similar process was used previously in constructing strains KSP122 (htrA S234A) and KSP148 (htrA ϩ ) in the same background (8). Competence Assays-A luciferase reporter consisting of an ssbBЈ-luc transcriptional fusion was used to measure activation of the competence system as described previously (8).…”

“…Notably, members of the HtrA family, in addition to serving as proteases, commonly function as chaperones as well (14,27,28). The repression of competence by pneumococcal HtrA has been shown to depend specifically on its protease activity (8,29), although the target through which it exerts this regulatory effect has not previously been determined.…”