Heterogeneity of the principal sigma factor in Escherichia coli: the rpoS gene product, sigma 38, is a second principal sigma factor of RNA polymerase in stationary-phase Escherichia coli.

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Abstract
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 (sigma 38) was overproduced using the cloned rpoS gene and purified to homogeneity. Reconstituted RNA polymerase holoenzyme (E sigma 38) was found to recognize in vitro a number of typical sigma 70-type promoters, including the lacUV5 and trp promoters. Some, however, were recognized exclusively or preferentially by E sigma 70, whereas at least one, fic, was favored by E sigma 38. Thus E. coli promoters can be classified into three groups: the first group is recognized by E sigma 70 and E sigma 38, but the second and third groups are recognized substantially by either E sigma 70 or E sigma 38 alone. In contrast to other minor sigma factors, sigma 38 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 sigma 38 was demonstrated to increase in vivo upon entry into stationary phase. These results together indicate that sigma 38 is a second principal sigma factor in stationary-phase E. coli.