CONSERVED FUNCTION AND REGULATION OF SIGMA(32) HOMOLOGS IN GRAM-NEGATIVE BACTERIA

The heat shock response in Escherichia coli and related bacteria is pr imarily mediated by sigma(32) or its homologue (RpoH protein) specific ally required for transcription of heat shock genes encoding molecular chaperones and proteases. Extensive work in E. coli revealed some of the mechanisms controlling the cellular level and activity of sigma(32 ) during the heat shock response. Recent isolation of a number of RpoH homologues from gamma, beta and alpha proteobacteria provided an oppo rtunity to examine evolutionary conservation and diversity of regulato ry mechanisms in these bacteria. We here summarize the present status of this aspect of the stress response not only by comparative sequence analysis but by examining the response of representative RpoH homolog ues of the gamma subgroup to heat shock stress. Current evidence indic ates that the basic strategy of enhancing RpoH level as a primary resp onse to heat shock stress is well conserved, but the detailed mechanis ms for enhancement of the heat shock a factor level vary among differe nt species that may reflect diverse ecological niches.