Carl Bauer Laboratory: Regulating The Bacterial Photosystem In Response To Changes In Light Intensity

Regulating The Bacterial Photosystem In Response To Changes In Light Intensity

The blue light regulated antirepressor, AppA

We recently described the existence of a blue light regulated transcription factor from Rhodobacter sphaeroides called AppA. AppA functions as an antirepressor of the redox controlled repressor called PpsR (PpsR is a homolog of CrtJ from R. capsulatus that is discussed in more detail the oxygen regulation section of our web site) (Masuda and Bauer 2002). Under dark or dim light conditions, AppA can tightly bind to PpsR and inhibit the biding of PpsR to target DNA sequences. However, when illuminated with blue light, then AppA is unable to bind to PpsR. The mechanism of light mediated regulation of AppA antirepressor activity is an active part of our research program and has been demonstrated by our group to involve light excitation of a flavin that is bound to AppA (Masuda and Bauer 2002)

We have undertaken x-ray crystallographic as well as time resolved fast spectroscopic analysis using laser excitation of AppA (in collaboration with Dr. Bogden Dragnea in the IU Chemistry department) to delineate the mechanism of blue light regulation of AppA.

Fig. 2. Crystal structure of AppA demonstrated a novel flavin-binding motif involving the partitioning of the cyclic ring of flavin between two helices flanked by a beta sheet (Anderson et al., 2004)

Fig. 3. Conserved hydrogen bond interactions present in the flavin binding pocket of AppA (Andersen et al., 2004)

Fig 4. Ultra fast laser pulse probe spectroscopic analysis of AppA reveals several intermediates such as a Trp and flavin radicals that are involved in initiating the photocycle upon light excitation of the flavin (Dragnea et al., 2005)

Comparative Study With The Cyanobacterial Photoreceptor Slr1694

Recently, we have undertaken a comparative approach toward understanding features of the AppA photocycle. For this analysis, we have obtained a crystal structure of the AppA related protein form cyanobacteria called Slr1694 This protein crystallizes as a stacked set of pentamer rings

Fig. 5. Structure of the cyanobacterial photoreceptor Slr694

Individual subunits of Slr1694 show extensive similarity to the amino terminal domain of AppA.

Fig 6. Overlay of the flavin-binding domain of AppA with Slr1694.

Currently we are undertaking crystallographic, mutational and spectroscopic analysis of AppA and Slr1694 to reveal features of the dark and light excited forms of these proteins. Our goal is to obtain an understudying of the molecular mechanism of the photocycle in this novel class of photoreceptors.

References :

Masuda, S. & C. E. Bauer. 2002. AppA is a blue-light photoreceptor that anti-represses photosynthesis gene expression in Rhodobacter sphaeroides Cell 110, 613-623. (Featured on cover)
Masuda, S. & C. E. Bauer 2005 The antirepressor AppA uses the novel flavin-binding BLUF domain as blue-light-absorbing photoreceptor to control photosystem synthesis. In Handbook of Photosensory Receptors. (W. Briggs and J. Spudich edt). Wiley-VCH publishing, Weinheim , Germany . pp. 433-446
Kraft, B., J. Kikuchi , Masuda, S., V. Dragnia, G. Tollin, J. M. Zaleski, & C. E. Bauer . 2003 . Spectroscopic and mutational analysis of the blue-light photoreceptor, AppA: A novel photocycle involving flavin stacking with aeromatic amino acids. Biochemistry , 42: 6726-6734
Anderson, S., V. Dragnea, V., S. Masuda, J. Ybe, K. Moffat & C. E. Bauer. 2005. Structure of a novel photoreceptor: the BLUF domain of AppA from Rhodobacter sphaeroides . Biochemistry . 44, 7998-8005
Dragnea , V., M. Waegelle, S. Balascuta, C. E. Bauer and B. Dragnea. 2005 Spectroscopic studies of the AppA blue-light receptor BLUF domain from Rb. sphaeroides in solution and in a crystal lattice with time-resolved spectroscopy . Biochemistry . 44:15978-15985.
Yuan, H., S. Masuda, V, Dragnea, S. Anderson , K. Moffat and C. E. Bauer. 2006. Structure of the blue light photoreceptor, BluP (Slr1694) from Synechocystis PCC6803 reveals photoinduced alterations of a hydrogen bond network to FAD. In Preparation.