Increased transcription rates correlate with increased reversion rates in leuB and argH Escherichia coli auxotrophs

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Abstract
Escherichia coliauxotrophs ofleuBandargHwere examined to determine if higher rates of transcription in derepressed genes were correlated with increased reversion rates. Rates ofleuBandargHmRNA synthesis were determined using half-lives and concentrations, during exponential growth and at several time points during 30 min of amino acid starvation. Changes in mRNA concentration were primarily due to increased mRNA synthesis and not to increased stability. Four strains ofE. coliamino acid auxotrophs, isogenic except forrelAandargR, were examined. In both theleuBandargHgenes, rates of transcription and mutation were compared. In general, strains able to activate transcription with guanosine tetraphosphate (ppGpp) had higher rates of mRNA synthesis and mutation than those lacking ppGpp (relA2mutants).argRknockout strains were constructed inrelA+andrelAmutant strains, and rates of bothargHreversion and mRNA synthesis were significantly higher in theargRknockouts than in the regulated strains. A statistically significant linear correlation between increased rates of transcription and mutation was found for data from both genes. In general, changes in mRNA half-lives were less than threefold, whereas changes in rates of mRNA synthesis were often two orders of magnitude. The results suggest that specific starvation conditions target the biosynthetic genes for derepression and increased rates of transcription and mutation.