Sensing Starvation: A Homoserine Lactone-Dependent Signaling Pathway in Escherichia coli

When nutrients become limiting, many bacteria differentiate and become resistant to environmental stresses. For Escherichia coli, this process is mediated by the σ$^s$ subunit of RNA polymerase. Expression of σ$^s$ was induced by homoserine lactone, a metabolite synthesized from intermediates in threonine biosynthesis. Homoserine lactone-dependent synthesis of σ$^s$ was prevented by overexpression of a newly identified protein, RspA. The function of homoserine lactone derivatives in many cell density-dependent phenomena and the similarity of RspA to a Streptomyces ambofaciens protein suggest that synthesis of homoserine lactone may be a general signal of starvation.