The common theme of our work is the biochemistry and evolution of photosynthesis.
Although we have a good understanding of the 'Z-scheme’ of photosynthesis, it is increasingly clear that there are many side-reactions that help protect the photosynthetic machinery against damage from excessive or rapidly fluctuating light. Some years ago, we discovered a novel protein, cytochrome c6A, which is related to the well-characterized cytochrome c6 that can transfer electrons from the cytochrome bf complex to Photosystem I in some algae. However, cytochrome c6A is unable to carry out this function because (among other reasons) its redox potential is unsuitable (1). Having discovered this protein, we want to find out what it does – it is highly conserved, suggesting an important function, and we think it may be involved in a photoprotection response.
We and others have shown that algae and photosynthetic bacteria emit electrons both in the dark and (more so) in the light. These electrons can be collected by an anode, and form the basis of 'biophotovoltaic’ systems, which we have shown can be used to power small electronic devices such as environmental sensors. Although the power densities achieved at present are lower than for conventional solar cells, biophotovoltaics should be cheaper to produce (and decommission!) and may be useful for power production in remote areas, perhaps in developing countries, or environmentally sensitive locations. We aim to understand how electrons are released from photosynthetic microorganisms, and how we might be able to increase electron output (3, 4). As well as their practical applications, biophotovoltaic systems may also be a useful tool for studying the electron transfer pathways of photosynthetic microorganisms.
There is increasing interest in the possibility of using purple anoxygenic photosynthetic bacteria, such as Rhodopseudomonas palustris, as a biotechnology chassis. We think it will be particularly valuable for waste treatment, as it is metabolically versatile, and resilient to many toxic materials (5). The combination of waste processing with production of useful molecules ('waste valorisation’) is particularly attractive, and we are developing systems to do this using R. palustris.
At the evolutionary level, we are interested in the evolution of the chloroplast genome. Most plants and algae have a chloroplast genome containing over a hundred genes. We were one of the first labs to report an unusual genome organization in dinoflagellates, an important group of algae forming part of the coral-algal symbiosis and responsible for red tides. Dinoflagellates have lost most of their chloroplast genes to the nucleus, and the remainder are located on small plasmids, typically containing a single gene. Primary transcripts from the whole plasmid are processed in various ways, including the addition of a polyU tail (6). We are interested in understanding more about how the genome is maintained and transcribed. We are also keen to develop the chloroplast genome as a genetic marker for studies on coral bleaching in which coral hosts expel their dinoflagellate symbionts, and to develop dinoflagellate chloroplast transformation systems
An important group of protist parasites, the Apicomplexa (which include Toxoplasma and Plasmodium, and are a sister group to the dinoflagellates) have a photosynthetic ancestry and surprisingly contain a remnant chloroplast, with a genome of 35 kbp (7, 8). The remnant chloroplast is essential for parasite survival, and we now know that some antimalarials, such as clindamycin, work by interfering with it. However, little is known about gene expression in the parasite chloroplast, how it is regulated, and how it is co-ordinated with expression of the nuclear genome. We aim to understand more about these important aspects of parasite biology, with a view to generating novel antimalarials.
1) Molina-Heredia FP, Wastl J, Navarro JA, Bendall DS, Hervas M, Howe CJ, De La Rosa MA (2003) 'A new function for an old cytochrome' Nature 424:33-34
2) Worrall JAR, Schlarb-Ridley BG, Reda T, Marcaida MJ, Moorlen RJ, Wastl J, Hirst J, Bendall DS, Luisi BF, Howe CJ (2007) 'Modulation of heme redox potential in the cytochrome c6 family' Journal of the American Chemical Society 129:9468-9475
3) Saar KL, Bombelli P, Lea-Smith DJ, Call T, Aro E-M, Mueller T, Howe CJ, Knowles TPJ (2018) “Enhancing power density of biophotovoltaics by decoupling storage and delivery” Nature Energy 3:75-81
4) McCormick AJ, Bombelli P, Bradley RW, Thorne R, Wenzel T, Howe CJ (2015) “Biophotovoltaics: oxygenic photosynthetic organisms in the world of bioelectrochemical systems” Energy & Environmental Science 8:1092-1109.
5) Pott RWM, Howe CJ, Dennis JS (2014) “The purification of crude glycerol derived from biodiesel manufacture and its use as a substrate by Rhodopseudomonas palustris to produce hydrogen” Bioresource Technology 152:464-470.
6) Dorrell RG, Howe CJ (2015) “Integration of plastids with their hosts: Lessons learned from dinoflagellates” Proc Natl Acad Sci USA 112:10247-10254.
7) Howe CJ (1992) 'Plastid origin of an extrachromosomal DNA molecule from Plasmodium, the causative agent of malaria' Journal of Theoretical Biology 158 199-205
8) Nisbet RER, Kurniawan DP, Bowers HD, Howe CJ (2016) “Transcripts in the Plasmodium apicoplast undergo cleavage at tRNAs and editing, and include antisense sequences” Protist 167:377-388.
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