Proteorhodopsin in Flavobacterium

Light-induced transcriptional responses associated with proteorhodopsin-enhanced growth in a marine flavobacterium

Ten years ago, Beja et al isolated a bacterial rhodopsin. Proteorhodopsin (PR) was discovered through metagenomic analyses of marine bacterioplankton genome fragments.  PRs were subsequently detected in many other marine bacteria. Since then additional studies have found PR’s in a wide range of bacteria. PR-containing marine bacteria have been recently cultured from a variety of marine environments. These isolates include members of Proteobacteria and Vibrionaceae. Laboratory experiments examining light-stimulated growth in some of these bacteria have been uncertain. Some studies have detected no significant light enhancement of either growth rates in PR-containing bacteria whilst light-enhanced growth rates have been  reported in one PR containing marine flavobacterium, Dokdonia sp. MED134 (Gomez-Consarnau et al., 2007). Additionally studies have suggested that some of marine flavobacteria populations have exhibited increased expression of the PR gene in the presence of light. Gomez- Consarnau et al (2010) demonstrated the enhanced long-term survival of PR-containing Vibrio cells in light, but not in darkness. The specific metabolic processes that cause PR enhanced growth or survival are not yet well understood due to inconsistent results reported from studies.

To increase our understanding of the photo physiology of PR containing flavo bacteria, Kimura et al exposed MED134 to variety of nutrient conditions in light and dark. The effect of light on growth in low nutrient concentrations and the effect of retinal biosynthesis inhibitors on light-enhanced growth were also tested in this study. In addition, the effects of sodium inhibitors on light-stimulated growth were also examined.

This study saw a significant influence of PR on growth rate at low carbon concentrations, and its lesser influence at higher carbon concentrations in light. These findings are concurrent with previous work of Gomez-Consarnau et al. (2007), which showed that light, has an impact on the growth of bacteria grown in low carbon conditions.

Gomez-Consarnau et al. (2007) demonstrated that MED134 had a higher expression of PR gene in the light than in the dark. Kimura’s results extend this and indicate that transcription of the entire PR photosystem is upregulated in the presence of light in this bacterium with the use of Transcriptomic studies.

To support the results that PR is important in light-stimulated growth in MED134, Cultures were set up with MPTA, a inhibitor in the retinal biosynthetic pathway. Kimura et al saw that MPTA prevented b-carotene generation, the precursor for retinal. Next, MED134 was grown in new cultures amended with MPTA. Cells incubated with MPTA, grew moderately in the presence of light. In contrast, MED134 incubated without MPTA in light, produced significantly higher yields, there were no differences found in the dark. The findings suggest that PR bound to retinal has a critical role in light-stimulated growth.

Transcriptomic analyses revealed that several genes, encoding for membrane transporters, exhibited significant upregulation in the presence of light, in particular Na-translocating NQR in light. This indicates the importance of the sodium ion gradient for transport functions in light. To test the importance of sodium ion exchange, Kimura et al performed growth experiments with HQNO, the inhibitor of NQR. Cells were incubated in cultures with and without HQNO. In cultures without HQNO, there was significant growth of MED134 in the light. Cell yields in cultures incubated with HQNO in the light were about three times less than those grown in the absence of inhibitor. These findings indicate that the Na-translocating NQR has a critical role in sodium pumping in light stimulated growth.

This study’s findings are concurrent with results previously reported by Gomez  but this study also expands these results to give us a more complete picture. I think this study provides a great example of just how far metagenomic analysis has come in the last 10 years.

 Hiroyuki Kimura,Curtis R Young,Asuncion Martinez,and Edward F DeLong. (2011). Light-induced transcriptional responses associated with proteorhodopsin-enhanced growth in a marine flavobacterium. Available: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3176510/. Last accessed 10/11/2012