Regulation of proteorhodopsin gene expression by nutrient limitation in the marine bacterium Vibrio sp. AND4

Abstract

Proteorhodopsin (PR), a ubiquitous membrane photoprotein in marine environments, acts as a light-driven proton pump and can provide energy for bacterial cellular metabolism. However, knowledge of factors that regulate PR gene expression in different bacteria remains strongly limited. Here, experiments with Vibrio sp. AND4 showed that PR phototrophy promoted survival only in cells from stationary phase and not in actively growing cells. PR gene expression was tightly regulated, with very low values in exponential phase, a pronounced peak at the exponential/stationary phase intersection, and a marked decline in stationary phase. Thus, PR gene expression at the entry into stationary phase preceded, and could therefore largely explain, the stationary phase light-induced survival response in AND4. Further experiments revealed nutrient limitation, not light exposure, regulated this differential PR expression. Screening of available marine vibrios showed that the PR gene, and thus the potential for PR phototrophy, is found in at least three different clusters in the genus Vibrio. In an ecological context, our findings suggest that some PR-containing bacteria adapted to the exploitation of nutrient-rich micro-environments rely on a phase of relatively slowly declining resources to mount a cellular response preparing them for adverse conditions dispersed in the water column.

Expression of holo-proteorhodopsin in Synechocystis sp. PCC 6803

“…The drop in PR content during the stationary phase may be in part due to the promoter we selected, because it is known that psbA2 expression and transcript stability are light-intensity dependent (68)(69)(70)(71)(72)(73), and so is heterologous expression driven by this promoter (74,75). It is worth noting that several of the recently discovered proteorhodopsins in 'chemotrophic', often psychrophilic, bacteria only contribute detectably to growth rate or cell yield when the cells are stressed and/or in stationary phase (6,(14)(15)(16)(18)(19)(20).…”

“…To show that PR is able to make a contribution to light-energy conversion in Synechocystis, we also made use of starvation/stressful conditions, just like it was initially done in E. coli (21) and in some of the organisms in which it is endogenously present (6,(14)(15)(16)(18)(19)(20). However, no fully consistent data have been obtained under these conditions so far.…”

“…Since then, PRs have turned out to be highly abundant in the oceans (6)(7)(8)(9), and organisms containing them, including a cyanobacterium (10) can be found in many other habitats as well (7,(11)(12)(13). In vivo experiments have shown that pumping of protons by PR can lead to increases in growth rate under nutrient-limited conditions (6,14,15), production of ATP (16), increased fixation of CO 2 (6,17), and/or survival under starvation or stress conditions (16,(18)(19)(20). However, these enhancements generally require nutrient-limitation or stress conditions before they exceed experimental error.…”

Marine Bacterial and Archaeal Ion-Pumping Rhodopsins: Genetic Diversity, Physiology, and Ecology

“…For example, there are Gammaproteobacteria genomes carrying PR genes clustering with Actinobacteria (81). Further, there is differentiation within lowerlevel taxa, such as the Gammaproteobacteria genera Vibrio and Glaciecola, where some members have PRs clustering with Alphaproteobacteria and others have PRs clustering with Gammaproteobacteria (63). Curiously, though, Bacteroidetes PRs cluster together and are consistently distinct from PRs in other taxa, such as Proteobacteria (Fig.…”

Winter diversity and expression of proteorhodopsin genes in a polar ocean

“…However, this may have been influenced by substrate availability: the former study used low organic carbon concentrations in their experiments, whereas the latter used concentrations that were two orders of magnitude higher. Recent studies show that nutrient availability is also important in regulating PR expression (Akram et al, 2013). Under oligotrophic conditions, increased PR expression and growth was observed in some PR carriers undergoing starvation (Gómez-Consarnau et al, 2010) and also in unamended seawater samples exposed to light-dark cycles (Lami et al, 2009).…”