Single-cell heterogeneity in ribosome levels and protein synthesis during nutrient starvation is driven by cAMP signaling

Ribosomes are central to protein synthesis and a frequent target for antibiotics. In fast-growing bacteria, the ribosome content is proportional to the growth rate; how ribosomes and protein synthesis are regulated during nutrient starvation remains poorly understood, particularly in single cells. To address this, we fluorescently labeled ribosomal proteins (RPs) in Salmonella and explored their variations and regulation in single cells. We show that the RP levels become heterogeneous during the transition to the stationary phase. Unexpectedly, cells with higher RP levels responded less to the induction of gene expression but accumulated more virulence gene products. Our work further reveals that adenosine 3′,5′-monophosphate (cAMP) signaling increases the heterogeneity of the levels of RPs and other gene products. Fluorescence dilution assay and proteomic analysis indicate that cAMP signaling promotes gene expression heterogeneity by directing proteome-wide adaptation, which enables growth heterogeneity, hence differential dilution of gene products during nutrient depletion.