Competence for Genetic Transformation in Streptococcus pneumoniae: Mutations in σ A Bypass the comW Requirement

Abstract

ABSTRACT Competence for genetic transformation in the genus Streptococcus depends on an alternative sigma factor, σ X , for coordinated synthesis of 23 proteins, which together establish the X state by permitting lysis of incompetent streptococci, uptake of DNA fragments, and integration of strands of that DNA into the resident genome. Initiation of transient accumulation of high levels of σ X is coordinated between cells by transcription factors linked to peptide pheromone signals. In Streptococcus pneumoniae , elevated σ X is insufficient for development of full competence without coexpression of a second competence-specific protein, ComW. ComW, shared by eight species in the Streptococcus mitis and Streptococcus anginosus groups, is regulated by the same pheromone circuit that controls σ X , but its role in expression of the σ X regulon is unknown. Using the strong, but not absolute, dependence of transformation on comW as a selective tool, we collected 27 independent comW bypass mutations and mapped them to 10 single-base transitions, all within rpoD , encoding the primary sigma factor subunit of RNA polymerase, σ A . Eight mapped to sites in rpoD region 4 that are implicated in interaction with the core β subunit, indicating that ComW may act to facilitate competition of the alternative sigma factor σ X for access to core polymerase.