HETEROGENEITY OF THE PRINCIPAL SIGMA-FACTOR IN ESCHERICHIA-COLI - THE RPOS GENE-PRODUCT, SIGMA(38)-, IS A 2ND PRINCIPAL SIGMA-FACTOR OF RNA-POLYMERASE IN STATIONARY-PHASE ESCHERICHIA-COLI

The rpoS gene of Escherichia coli encodes a putative RNA polymerase sigma factor that is considered to be the central regulator of gene expression in stationary phase. The gene product (sigma38) was overproduced using the cloned rpoS gene and purified to homogeneity. Reconstituted RNA polymerase holoenzyme (Esigma38) was found to recognize in vitro a number of typical sigma70-type promoters, including the lacUV5 and trp promoters. Some, however, were recognized exclusively or preferentially by Esigma70, whereas at least one, fic, was favored by Esigma38. Thus E. coli promoters can be classified into three groups: the first group is recognized by Esigma70 and Esigma38, but the second and third groups are recognized substantially by either Esigma70 or Esigma38, alone. In contrast to other minor sigma factors, sigma38 shares a set of amino acid sequences common among the principal sigma factors of eubacteria and is therefore a member of the RpoD-related protein family. The intracellular level of sigma38 was demonstrated to increase in vivo upon entry into stationary phase. These results together indicate that sigma38 is a second principal sigma factor in stationary-phase E. coli.