Homocysteine thiolactone from translation proofreading is an endogenous ligand for cell-cell signaling by the receptor SdiA

ABSTRACT Several widespread mechanisms enable cellular communication and coordinate the behavior of bacterial populations. The modules responsible for cellular communication are often found in matching pairs: an enzyme that synthesizes a signal molecule and a specific receptor that decodes it. A notable exception is Escherichia coli SdiA, a homolog of known N-acyl-homoserine lactone (AHL) receptors, for which no corresponding AHL synthase exists. SdiA is an orphan receptor that enables cross-talk by sensing AHLs from other bacterial species. In this work, we investigated whether homocysteine thiolactone (HTL), an AHL-like molecule arising from a proofreading reaction of methionyl tRNA synthetase (MetRS) to correct misactivation of homocysteine, participates in E. coli communication through SdiA. A comprehensive analysis of MetRS variants producing different levels of HTL revealed more extended lag periods at lower HTL levels, which were restored by HTL supplementation. SdiA mutants abolished the HTL-induced regulation of the transition from the lag to the exponential phase. Growth phenotypes were HTL specific and were not induced by supplementation of other non-endogenous AHLs previously characterized as SdiA ligands. The gene expression of gadY and rmf , regulated by SdiA, was modulated by HTL supplementation. Through differential scanning fluorimetry, we show the SdiA-HTL interaction in vitro . Together, our observations describe HTL as an SdiA ligand that modulates cell-cell communication in bacteria. IMPORTANCE Bacteria need to adapt to different environments and stress conditions, and quorum sensing (QS) is a widespread and well-established mechanism that enables cell-to-cell communication to coordinate these changes. The modules responsible for QS usually evolve in pairs: an enzyme that synthesizes the signal and a specific receptor that decodes it. A notable exception is Escherichia coli SdiA, a homolog of known N-acyl-homoserine lactone (AHL) receptors for which no corresponding AHL synthase exists. It has been hypothesized that SdiA is an orphan receptor that enables crosstalk by sensing AHLs from other bacterial species. We present data suggesting that homocysteine thiolactone, a ubiquitous molecule arising from homocysteine editing during translation proofreading, can mimic AHLs and participate in bacterial cell-cell communication via SdiA. The near-universal distribution of this proofreading mechanism suggests that homocysteine thiolactone provides a common link between protein synthesis and cell-cell communication in both homologous and heterogeneous microbial populations.