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International Symposium on Photosynthetic Prokaryotes (2000)
Meeting Report
What's new in electron transport?
by Jason Cooley (Arizona State University)

The recent ISPP meeting saw not just the examination of known model systems like Synechocystis and Rhodobacter for biophysical and physiological function, but we also heard and read presentations on less well characterized organism like thermophilic cyanos and such interesting organisms as Roseobacter, an aerobically phototrophic purple bacterium. Of particular interest to me were several presentations that involved the structure and function of cytochrome-containing electron transport chain components and how they interact with and are regulated by the redox state of the aqueous and lipid-soluble mobile carrier pool(s) (i.e. the PC/cyt and PQ pool in cyanos).

The most expansive of these presentations came from work presented by Davide Zannoni, which characterized part of the electron transport chain of the purple aerobically phototrophic bacterium Roseobacter denitrificans. All three hemes of a cytochrome bc1 complex could be spectroscopically resolved. Furthermore, redox titrations of these spectral features indicate that the BL heme has a very low potential compared to that of Rhodobacter. Two other hemes were spectrally resolved, with redox potentials of +218 and +335 mV, indicating the presence of an aa3 type cytochrome oxidase. When the O2 consumption was titrated with KCN, the enzyme had a Km more similar to that of a mitochondrial enzyme (Km of ~10 mM) than the more closely related Rhodobacter enzyme (Km of 50 mM). From oxygen evolution data electron transport did not seem to be branched and proceeded to a single terminal oxidase. However, based on measurements with a quinone electrode that there is an apparent contribution from quinol oxidase(s) when the UQ pool is more than a quarter reduced. It can then be concluded that the quinol oxidases may help to poise the quinone pool within an "optimal" range under more reducing conditions -- possibly a lesson for other organisms and other less well understood protein complexes!

Chris Nomura, from Don Bryant's lab, described the knocking out of several electron transport genes from Synechococcus sp. PCC 7002 and the different gene organization in that cyanobacterium compared to that of Synechocystis PCC 6803. Among the genes knocked out was a gene cluster for a putative cytochrome oxidase that upon inactivation led to stress under extremely high light conditions.

Judith Armitage gave a very nice synopsis of the work from her lab, outlining that chemo-, photo-, and aerotaxis regulation all go through the classical Che system of signal transduction. This system is under the control of the redox sensory PrrA/B pathway, which also controls expression of photosynthesis and carbon fixation genes. Of particular interest here was that this regulation is dependent upon electron transport and not upon a proton motive force or the presence of accessory pigments. This indicates that the controlling entity may be related to the redox state of the Q pool.

Christopher Howe gave a nice talk detailing work in his lab that helps us understand the interaction of plastocyanin and cytochrome f in plants versus cyanos. The primary point was that the a-band shift that is observed in spectra from the cytochrome f from plants vs. cyanos can be attributed to a difference in a single residue of the N-terminal region of the protein. However, no clear explanation or hypothesis of the mechanism of interaction or binding was given for the cyanobacterial plastocyanin/cytochrome f scenario.

The meeting provided not only interesting work, but also gave us a taste of some of the approaches and ideas about electron transport systems that will be important in future research.