A new mutation rpoD800, affecting the sigma subunit of E. coli RNA polymerase is allelic to two other sigma mutants.

We have characterized a new mutation rpoD800 affecting the sigma gene of E. coli. Upon tranfer to high temperature, a strain with the rpoD800 mutation ceases growth within 30 min. We find that this mutation renders sigma about 10-fold more thermolabile than the wild type sigma at 45 degrees C in vitro. We have compared the temperature profile for inactivation of wild type and mutant sigma and find that the mutant inactivates at a temperature about 9 degrees C lower than does the wild type. The chromosomal locus affected by rpoD800 is shown to be allelic to the locus affected by the spontaneous mutants ts285 and alt-1. All three mutations result in altered sigma and in altered growth at high temperature. We argue that the single locus affected is the structural gene for the sigma subunit of E. coli RNA polymerase.