Identification and structure of the anti-sigma factor-binding domain of the disulphide-stress regulated sigma factor sigma(R) from Streptomyces coelicolor.
The extracytoplasmic function (ECF) sigma factor sigma(R) is a global regulator of redox homeostasis in the antibiotic-producing bacterium Streptomyces coelicolor, with a similar role in other actinomycetes such as Mycobacterium tuberculosis. Normally maintained in an inactive state by its bound anti-sigma factor RsrA, sigma(R) dissociates in response to intracellular disulphide-stress to direct core RNA polymerase to transcribe genes, such as trxBA and trxC that encode the enzymes of the thioredoxin disulphide reductase pathway, that re-establish redox homeostasis. Little is known about where RsrA binds on sigma(R) or how it suppresses sigma(R)-dependent transcriptional activity. Using a combination of proteolysis, surface-enhanced laser desorption ionisation mass spectrometry and pull-down assays we identify an N-terminal, approximately 10kDa domain (sigma(RN)) that encompasses region 2 of sigma(R) that represents the major RsrA binding site. We show that sigma(RN) inhibits transcription by an unrelated sigma factor and that this inhibition is relieved by RsrA binding, reaffirming that region 2 is involved in binding to core RNA polymerase but also demonstrating that the likely mechanism by which RsrA inhibits sigma(R) activity is by blocking this association. We also report the 2.4A resolution crystal structure of sigma(RN) that reveals extensive structural conservation with the equivalent region of sigma(70) from Escherichia coli as well as with the cyclin-box, a domain-fold found in the eukaryotic proteins TFIIB and cyclin A. sigma(RN) has a propensity to aggregate, due to steric complementarity of oppositely charged surfaces on the domain, but this is inhibited by RsrA, an observation that suggests a possible mode of action for RsrA which we compare to other well-studied sigma factor-anti-sigma factor systems.
Li
Wei
W
School of Biological Sciences, University of East Anglia, NR4 7TJ, Norwich, UK.
Stevenson
Clare E M
CE
Burton
Nicolas
N
Jakimowicz
Piotr
P
Paget
Mark S B
MS
Buttner
Mark J
MJ
Lawson
David M
DM
Kleanthous
Colin
C
D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D012321:DNA-Directed RNA Polymerases / Q000378:metabolism; D004220:Disulfides / Q000378:metabolism*; D013058:Mass Spectrometry; D008667:Metalloproteins / Q000235:genetics / Q000378:metabolism; D008958:Models, Molecular; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D012808:Sigma Factor / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D013302:Streptomyces / Q000737:chemistry* / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism
D001426:Bacterial Proteins; D004220:Disulfides; D008667:Metalloproteins; C121147:RsrA protein, Streptomyces coelicolor; D012808:Sigma Factor; D014157:Transcription Factors; D012321:DNA-Directed RNA Polymerases