Stressosomes; Where it all began.


Big ball of stress…. That’s me. Studying a tiny stress sensing ball – the stressosome. Brilliant name but what is it?

The stressosome is a multiprotein, stress sensing complex, composed of proteins RsbR, RsbS and RsbT, in Bacillus subtillis, which is the model organism for stressosome study. The function of the stressosome has been shown to be the activation of an alternative transcription factor,  SigB, with the ultimate goal of generating a general stress response (GSR) in response to environmental stressors including ethanol, temperature and fluctuations in pH or salinity 1,2.

In B.subtilis, the stressosome proteins RsbR, RsbS both function as scaffold proteins, which – in the absence of stress – hold RsbT, a positive regulator of SigB , in an inactive state. Upon stress, which is probably sensed by RsbR, RsbT is released from the surface of the RsbR:RsbS complex and conveys the stress signal in a signal cascade including additional proteins, ultimately activating the alternative sigma factor, SigB. Consequently, all proteins within this signaling cascade were named  Regulator of Sigma B (RsbR)1,2 (Figure 1). But this information, as straight forward as it looks, didn’t arrive overnight. It took years to determine. And a lot is still unknown (pause for dramatic effect).

Although the proteins forming the stressosome were already discovered  1996, it took almost an addition 10 years to realize that they associate to form a huge (1.6 MDa) protein complex (). But this discovery was due to years of extensive research of SigB regulation – the stressosome discovery was a 2 birds 1 stone scenario.

Interest on SigB regulation began in the early ‘90s, which paved the way to the discovery of the stressosome. After initial interest declined in SigB (identified by the Losick group at Harvard university), after it was found it was not to be directly involved in sporulation.


“Started from the bottom now we’re here”


The SigB operon consists of 8 genes – RsbR-RsbS-RsbT-RsbU-RsbV-RsbW-SigB-RsbX, with the discovery of the regulatory sequences being identified from the bottom up of the operon (left to right).

The Haldenwang group in tandem with the Price group (University of California Davis) published extensive work on the regulation within the SigB operon (RsbV-RsbW-SigB-RsbX). Their findings, which formed the basis for the stressosome regulatory model today, eluded from biochemical and genetic experiments that RsbW was the main, negative, regulator of SigB, whereas the other 2 members of the SigB operon, RsbV and RsbX, were found to be a positive regulator and a negative regulator of SigB, respectively5,6. The Haldenwang group also showed the mode of regulation of SigB by RsbW and RsbV. RsbW was shown to occlude binding of SigB to the RNA polymerase complex by binding to SigB, thus blocking transcription of SigB dependent genes 7.

Additionally, they found the mode of regulation for the positive regulator of SigB, RsbV, the anti-anti-sigma factor (all in the name, stops RsbW, anti-sigma factor, from binding to SigB).  It was found that a non-phosphorylated variant of RsbV, bound to RsbW, prohibited the formation of the inhibitory RsbW-sigB complex8.

Price and colleagues, went on to discover the importance of the upstream counterparts of the SigB operon, now named RsbR-RsbS-RsbT-RsbU (Tying it all together). First identifying RsbU as a positive regulator of SigB in the general stress response9 they noted the regulatory importance of the upstream genes RsbR-RsbS and RsbT. Finding RsbR-RsbS and RsbT to be negative and positive regulators of SigB, respectively, and importantly that these regulatory proteins required RsbW and RsbV to exert their regulatory function, showing that the regulatory signal propagated throughout the operon and indeed these proteins were included in the SigB regulated signal cascade. So now, we know, in the case of B.subtilis that RsbU is required to dephosphorylate (activate) the anti-anti-sigma factor RsbVand RsbT activates RsbU upon release from the stessosome. Structural data then followed for stressosome proteins, a big contributer being  a member of the PATHSENSE project, Rick Lewis. This group were able to visualize the stressosome on an atomic level and provide detailed information on the structure of the ternary RsbR-RsbS-RsbT stressosome complex.10


The big picture…

Fig. 1 The intricately regulated SigB dependent General Stess Response, activated by the Stessosome.

In sensing environmental stress by RsbR, RsbT phosphorylates RsbR and RsbS (blue circles) resulting in RsbT release from the stressosome and activation of RsbU, a phosphatase which dephosphorylates RsbV. Dephosphorylated RsbV has a higher affinity for RsbW than SigB, this results in RsbW to form a complex with RsbV instead of SigB. SigB is then free to direct transcription of General Stress Response genes. RsbX, a phosphatase, resets the stressosome to its original state. Figure adapted from 1


So, a vast amount of knowledge is known about the stressosome in B. subtillis, however, little is known in the myriad of bacteria containing variants of stressosome proteins. Enter Pathsense (making sense of pathogens – brilliant pun) and the research on this fascinating subject continues.

Watch this space.



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  7. Benson, A. K. & Haldenwang, W. G. Bacillus subtilis sigma B is regulated by a binding protein (RsbW) that blocks its association with core RNA polymerase. Proc. Natl. Acad. Sci. U. S. A. 90, 2330–2334 (1993).
  8. Dufour, A. & Haldenwang, W. G. Interactions between a Bacillus subtilis anti-sigma factor (RsbW) and its antagonist (RsbV). J. Bacteriol. 176, 1813–1820 (1994).
  9. Wise, A. A. & Price, C. W. Four additional genes in the sigB operon of Bacillus subtilis that control activity of the general stress factor sigma B in response to environmental signals. J. Bacteriol. 177, 123–133 (1995).
  10. Marles-Wright, J. et al. Molecular architecture of the ‘stressosome,’ a signal integration and transduction hub. Science 322, 92–96 (2008).