The cyanobacterial principal σ factor region 1.1 is involved in DNA-binding in the free form and in transcription activity as holoenzyme

Cyanobacterial principal sigma factor, sigma(A), includes a specifically conserved cluster of basic amino acids in the amino-terminal extension called region 1.1. We found that the sigma(A) in a thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 binds DNA in the absence of the core RNA polymerase and that sigma(A) lacking region 1.1 is not able to bind DNA. This indicates that, in the cyanobacterium, region 1.1 participates in DNA-binding, rather than inhibiting the interaction between free alpha and DNA, as found in other principal sigma factors of eubacteria. The results of in vitro transcription assays with the reconstituted RNA polymerase showed that region 1.1 reduces transcription activity from the cpc promoter. (c) 2006 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.