Genes regulated by the Escherichia coli SOS repressor LexA exhibit heterogenous expression

Abstract

Abstract

Background: Phenotypic heterogeneity may ensure that a small fraction of a population survives environmental perturbations or may result in lysis in a subpopulation, to increase the survival of siblings. Genes involved in DNA repair and population dynamics play key roles in rapid responses to environmental conditions. In Escherichia coli the transcriptional repressor LexA controls a coordinated cellular response to DNA damage designated the SOS response. Expression of LexA regulated genes, e.g. colicin encoding genes, recA , lexA and umuDC , was examined utilizing transcription fusions with the promoterless gfp at the single cell level.

Results: The investigated LexA regulated genes exhibited heterogeneity, as only in a small fraction of the population more intense fluorescence was observed. Unlike recA and lexA , the pore forming and nuclease colicin activity genes as well as umuDC , exhibited no basal level activity. However, in a lexA defective strain high level expression of the gene fusions was observed in the large majority of the cells. All of the investigated genes were expressed in a recA defective strain, albeit at lower levels, revealing expression in the absence of a spontaneous SOS response. In addition, the simultaneous expression of cka , encoding the pore forming colicin K, and lexA , investigated at the single cell level revealed high level expression of only cka in rare individual cells.

Conclusion: LexA regulated genes exhibit phenotypic heterogeneity as high level expression is observed in only a small subpopulation of cells. Heterogenous expression is established primarily by stochastic factors and the binding affinity of LexA to SOS boxes.