Functional identification of an anti-σE factor from Thermus thermophilus HB8

The TTHB212 gene from extremely thermophilic bacterium Thermus thermophilus HB8 forms an operon with the upstream sigE gene encoding an extracytoplasmic function σ factor, σE, the sole alternative σ factor of this strain, on megaplasmid pTT27. The TTHB212 gene encodes a poorly conserved protein, which has been predicted to be a transmembrane one with N-terminal intracellular and C-terminal extracytoplasmic domains. The N-terminal domain of TTHB212 protein (TTHB212N) prevented σE from binding to RNA polymerase (RNAP) core enzyme in vitro, and TTHB212N bound σE in a molar ratio of 1:1 when both proteins were co-expressed in Escherichia coli. Furthermore, TTHB212N inhibited the transcription activity of RNAP-σE holoenzyme, but not that of the RNAP-σA one, in vitro. The expression of several genes that are under the control of σE was increased in a TTHB212 gene-disruptant strain. Thus, TTHB212 protein was identified as an anti-σE factor. These findings indicate that T. thermophilus HB8 has a regulatory system involving σE and anti-σE factors.