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Characterization of a (p)ppApp Synthetase Belonging to a New Family of Poly

Polymorphic toxins (PTs) are a broad family of toxins involved in interbacterial competition and pathogenesis. PTs are modular proteins that are comprised of a conserved N-terminal domain responsible for its transport, and a variable C-terminal domain bearing toxic activity. Although the mode of transport has yet to be elucidated, a new family of putative PTs containing an N-terminal MuF domain, resembling the Mu coliphage F protein, was identified in prophage genetic elements. The C-terminal toxin domains of these MuF PTs are predicted to bear nuclease, metallopeptidase, ADP-ribosyl transferase and RelA_SpoT activities. In this study, we characterized the MuF-RelA_SpoT toxin associated with the temperate phage of Streptococcus pneumoniae SPNA45. We show that the RelA_SpoT domain has (p)ppApp synthetase activity, which is bactericidal under our experimental conditions. We further determine that the two genes located downstream encode two immunity proteins, one binding to and inactivating the toxin and the other detoxifying the cell via a pppApp hydrolase activity. Finally, based on protein sequence alignments, we propose a signature for (p)ppApp synthetases that distinguishes them from (p)ppGpp synthetases.

 

Comments:

This study seems to delve into the fascinating world of polymorphic toxins (PTs) and specifically focuses on the characterization of a particular PT associated with the temperate phage of Streptococcus pneumoniae SPNA45. The identified PT in this case, MuF-RelA_SpoT, exhibits (p)ppApp synthetase activity, which has a bactericidal effect under the experimental conditions.

The research also sheds light on the genes located downstream, which encode two immunity proteins. One of these proteins binds to and deactivates the toxin, while the other detoxifies the cell by utilizing a pppApp hydrolase activity. This process suggests a protective mechanism in the bacterial cell against the potentially harmful effects of these toxins.

Moreover, the study proposes a distinctive signature for (p)ppApp synthetases based on protein sequence alignments. This signature is intended to differentiate these synthetases from (p)ppGpp synthetases, potentially aiding in their identification and classification.

The findings not only contribute to understanding the function and mechanism of PTs but also provide insights into the intricate interplay between bacteriophages, bacteria, and their defense mechanisms.

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