Molecular and Structural Characterization of the PezAT Chromosomal Toxin-Antitoxin System of the Human Pathogen Streptococcus pneumoniae*

The chromosomal pezT gene of the Gram-positive pathogen Streptococcus pneumoniae encodes a protein that is homologous to the zeta toxin of the Streptococcus pyogenes plasmid pSM19035-encoded epsilon-zeta toxin-antitoxin system. Overexpression of pezT in Escherichia coli led to severe growth inhibition from which the bacteria recovered ∼3 h after induction of expression. The toxicity of PezT was counteracted by PezA, which is encoded immediately upstream of pezT and shares weak sequence similarities in the C-terminal region with the epsilon antitoxin. The pezAT genes form a bicistronic operon that is co-transcribed from a σ70-like promoter upstream of pezA and is negatively autoregulated with PezA functioning as a transcriptional repressor and PezT as a co-repressor. Both PezA and the non-toxic PezA2PezT2 protein complex bind to a palindrome sequence that overlaps the promoter. This differs from the epsilon-zeta system in which epsilon functions solely as the antitoxin and transcriptional regulation is carried out by another protein designated omega. Results from site-directed mutagenesis experiments demonstrated that the toxicity of PezT is dependent on a highly conserved phosphoryltransferase active site and an ATP/GTP nucleotide binding site. In the PezA2PezT2 complex, PezA neutralizes the toxicity of PezT by blocking the nucleotide binding site through steric hindrance.

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