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Conjugative transfer of the integrative and conjugative element ICE clc  in the bacterium  Pseudomonas knackmussii  is the consequence of a bistable decision taken in some 3% of cells in a population during stationary phase. Here we study the possible control exerted by the stationary phase sigma factor RpoS on the bistability decision. The gene for RpoS in  P. knackmussii  B13 was characterized, and a loss-of-function mutant was produced and complemented. We found that, in absence of RpoS, ICE clc  transfer rates and activation of two key ICE clc  promoters (P  int   and P  inR  ) decrease significantly in cells during stationary phase. Microarray and gene reporter analysis indicated that the most direct effect of RpoS is on P  inR  , whereas one of the gene products from the P  inR  -controlled operon (InrR) transmits activation to P  int   and other ICE clc  core genes. Addition of a second  rpoS  copy under control of its native promoter resulted in an increase of the proportion of cells expressing the P  int   and P  inR   promoters to 18%. Strains in which  rpoS  was replaced by an  rpoS-mcherry  fusion showed high mCherry fluorescence of individual cells that had activated P  int   and P  inR  , whereas a double-copy  rpoS-mcherry –containing strain displayed twice as much mCherry fluorescence. This suggested that high RpoS levels are a prerequisite for an individual cell to activate P  inR   and thus ICE clc  transfer. Double promoter–reporter fusions confirmed that expression of P  inR   is dominated by extrinsic noise, such as being the result of cellular variability in RpoS. In contrast, expression from P  int   is dominated by intrinsic noise, indicating it is specific to the ICE clc  transmission cascade. Our results demonstrate how stochastic noise levels of global transcription factors can be transduced to a precise signaling cascade in a subpopulation of cells leading to ICE activation.

Cellular Variability of RpoS Expression Underlies Subpopulation Activation of an Integrative and Conjugative Element