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Crystal structure of hypothetical protein TTHB210, controlled by the σE/anti‐σE regulatory system in Thermus thermophilus HB8, reveals a novel homodecamer

The bacterial RNA polymerase (RNAP) core enzyme, with the subunit structure a2bb 0x, binds r factor, which is responsible for promoter recognition and selectivity. Most bacteria have a primary r factor that recognizes the majority of the genes, and several alternative r factors that recognize the promoters of specialized regulons, such as those involved in growth transitions, morphological changes, responses to environmental stress conditions, and pathogens.1 Extracytoplasmic function (ECF) r factors are alternative r factors, and many of them control the transcription of various genes involved in cell envelope functions and extracytoplasmic stress conditions.2 In many cases, an ECF r factor is co-transcribed with a transmembrane anti-r factor with an extracytoplasmic sensor domain and an intracellular inhibiting domain, which negatively regulates the activity of the ECF r factor.3 As long as the cognate environmental signal is absent, the ECF r factor is held by the anti-r factor in an inactive complex, which dissociates to activate the r factor when the anti-r factor senses the appropriate environmental signal. The extremely thermophilic bacterium, Thermus thermophilus HB8, has the ECF r factor family protein r as its sole alternative r factor.4 This strain has also the typical anti-r factor, which negatively regulates the activity of r.5 The r regulates five genes under three promoters on the megaplasmid pTT27.4,5 The r-regulated gene products, besides itself and anti-r factor, are hypothetical (or functionally unknown) proteins; therefore, the physiological function of the r/anti-r regulatory system has remained elusive. In this study, we determined the X-ray crystal structure of one of the r-regulated gene products, the TTHB210 protein, and found that it forms a novel homodecamer in the crystal, whereas it is a dimer in solution.