PR and Opportunitrophs- the hybrid cars among the microbes

Proteorhodopsins were only discovered in the last decade, they are light driven proton pumps which produce ATP via photophosphorylation. Genomic studies have revealed a huge range of unculturable or genetically hard to work with marine microbes, and more recently viruses, possess genes which code for proteorhodopsins (PR). However, gemonics alone cannot explore the function and physiology of proteorhodopsins in situ, furthermore just because a gene is possessed doesn’t mean it will always be expressed. Recently two culturable Vibrio species have been found to possess the pR genes; one strain (Vibrio AND4) was investigated by Gómez-Consarnau et al. (2010) which Matt has reviewed in an earlier blog, and the other (Vibrio campbellii) was investigated by the current paper reviewed in this blog.

Similar to Gómez-Consarnau et al. (2010), the authors report lots of data about the pR gene locus, sequence and phylogeny as well as exploring the function of the gene by inserting it into E.coli. They then go on to create a PR deficient mutant by knocking out the pR gene, this allows them to unpick the function of the gene and explore what fitness advantage it may give to the Vibrio using comparison experiments. The authors convincingly demonstrated that normal V. campbellii expressed pR genes (using q-PCR) and that the strain was absorbing green light and carrying out photophosphorylation to produce ATP, whereas the PR deficient mutant wasn’t. Perhaps surprisingly, under starvation conditions normal cells had the same survival rate as the PR deficient mutants; however, when a respiratory stress was introduced (azide, a respiratory chain poison), the normal cells showed a greater survival than the PR deficient mutants. 

By exposing both normal and mutant vibrios to either continuous illumination or darkness, Wang et al. investigated the role of light on the growth and pR expression of both strains. Under both conditions cell numbers, monitored by optical density measurements and flow-cytometry, peaked after 24 hours before declining. However, unlike previous studies, the decrease in the light treatment was much more pronounced than in the dark treatment. The authors suggested that this divergence was either due to the accumulation of deleterious effects of the continuous illumination (ROS production etc) or the induction of a lytic phage, since this specific Vibrio is known to harbour lysogenic viruses in its genome. 

The peak of pR expression in both light and dark treatments coincided with the transition of the culture to the stationary phase, suggesting that light was not the sole regulating factor and that possibly a sigma factor regulating the pathway linked to stress experienced during the stationary phase, namely the sigma factor RpoS, could be involved.  To test this Wang et al. created mutants lacking the rpoS gene, which were again exposed to either continuous light or dark. The normal transcripts of pR were significantly more numerous than in strain lacking rpoS, which strongly suggests that the RpoS sigma factor positively upregulates proteorhodopsin expression.  Since a small number of pR transcripts were measured in the rpoS deficient strain, it is also likely there is a second mechanism is involved in the regulation of PR. 

This study confirms the role and function of proteorhodopsin in Vibrios, as previously described by Gómez-Consarnau et al. (2010), and provides the first insight into the regulation of proteorhodopsin by the sigma factor RpoS. Similar hybrid car, it appears the solar powered ATP pump kicks in and provides a boost to help keep the cell ticking over and survive in times of stress. The term “opportuni-troph” attributed to V. campbellii seems very suitable; the pR gene, acquired via lateral transfer (also see Think Pink post), allows the exploitation of environments and energy sources unavailable for those lacking PR and, hence, may confer an evolutionary advantage in adverse conditions.  

References:

Gómez-Consarnau. L, Neelam.  A, Kristoffer. L, Pederson. A, Neutez. R, Milton. D. L, Gonzalez J. M, Pinhasi. J., (2010). Proteorhodopsin Phototrophy Promotes Survival of Marine Bacteria During Starvation. PLoS Biol 8(4): e1000358. doi:10.1371/journal.pbio.1000358.   

http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1000358

Wang Z, O'Shaughnessy TJ, Soto CM, Rahbar AM, Robertson KL, et al. (2012) Function and Regulation of Vibrio campbellii Proteorhodopsin: Acquired Phototrophy in a Classical Organoheterotroph. PLoS ONE 7(6): e38749. doi:10.1371/journal.pone.0038749

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0038749

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