The constancy of global regulation across a species: the concentrations of ppGpp and RpoS are strain-specific in <it>Escherichia coli</it>

<p>Abstract</p> <p>Background</p> <p>Sigma factors and the alarmone ppGpp control the allocation of RNA polymerase to promoters under stressful conditions. Both ppGpp and the sigma factor σ^S(RpoS) are potentially subject to variability across the species <it>Escherichia coli</it>. To find out the extent of strain variation we measured the level of RpoS and ppGpp using 31 <it>E. coli </it>strains from the ECOR collection and one reference K-12 strain.</p> <p>Results</p> <p>Nine ECORs had highly deleterious mutations in <it>rpoS</it>, 12 had RpoS protein up to 7-fold above that of the reference strain MG1655 and the remainder had comparable or lower levels. Strain variation was also evident in ppGpp accumulation under carbon starvation and <it>spoT </it>mutations were present in several low-ppGpp strains. Three relationships between RpoS and ppGpp levels were found: isolates with zero RpoS but various ppGpp levels, strains where RpoS levels were proportional to ppGpp and a third unexpected class in which RpoS was present but not proportional to ppGpp concentration. High-RpoS and high-ppGpp strains accumulated <it>rpoS </it>mutations under nutrient limitation, providing a source of polymorphisms.</p> <p>Conclusions</p> <p>The ppGpp and σ^Svariance means that the expression of genes involved in translation, stress and other traits affected by ppGpp and/or RpoS are likely to be strain-specific and suggest that influential components of regulatory networks are frequently reset by microevolution. Different strains of <it>E. coli </it>have different relationships between ppGpp and RpoS levels and only some exhibit a proportionality between increasing ppGpp and RpoS levels as demonstrated for <it>E. coli </it>K-12.</p>