The molar ratio of sigma 73 to core polymerase in the obligate intracellular bacterium, Rickettsia prowazekii.

In the obligate intracellular parasitic bacterium, Rickettsia prowazekii, the molar ratio of sigma 73 to core RNA polymerase, that is, the degree of saturation of the core polymerase by the catalytically active sigma factor, was very low. This ratio was determined from the radioactivity in rickettsial RNA polymerase immunoprecipitated from crude extracts of infected L929 cells in which the parasite was exponentially growing. If we assume that, as is true for the sigma subunit, in R. prowazekii and Escherichia coli the beta', and beta subunits of the RNA polymerase have similar methionine and cysteine contents (the radiolabelled amino acids), the molar ratio of sigma 73 to core polymerase in R. prowazekii would be 0.1. This is in striking contrast to E. coli where the ratio is typically 0.4. It remains to be established whether this low sigma saturation results in a limitation of active RNA polymerase in R. prowazekii and contributes to its slow growth.