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

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 synthesizing a signal molecule and a specific receptor capable of decoding 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 if 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 showed more extended lag periods due to lower HTL levels, which were restored by HTL supplementation. SdiA mutants abolished the regulation of the transition from the lag phase to the exponential phase induced by HTL. 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 capable of modulating cell-cell communication in bacteria.