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

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

Bacteria and Archaea possess Proteins that convert light in to energy. This paper is specifically concentrating on proteorhodopsin (PR), this protein consists of a transmembrane bound to a retinal molecule and functions as a light-driven proton pump. 10 years ago using the technique of metagenomics scientists concentrated on areas of the pacific ocean and Antarctica and stumbled across PR and now years later our knowledge is greater and this protein is found much further afield around the globe. These photoproteins tend to be in planktonic organisms in surface waters and  thought to be specifically adapted to absorb certain wave lengths of light of which is dependent on the habitat that the plankton are living in.

In a study previously performed (Gomez et al 2010) it was suggested that certain strains of flavobacterium could last longer in the light but not in the dark with the absence of nutrients (C) showing that the use of PR could be the factor allowing for not only survival but growth.

In Kimura et al 2011 their study focused on Dokdonia donghaensis, a flavobacteria  strain  MED134, they wanted to see how light affected proteorhodopsin, and how it changed growth rates MED134. The first of their tests was its cultivation in light and darkness where results showed when exposed to light in standard media the abundance was 1.1 × 10⁵cellsml⁻¹ but when the media was enriched with 0.14 m of carbon the result showed a growth of  1.4 × 10⁶cells ml⁻¹  further with  0.39m C added to the media a further growth was witnessed at  1.1 × 10⁷ . Then the MED134 cultures were subjected to dark and stayed under  5.0 × 10⁴cellsml⁻¹  and grew moderately throughout.   

This study showed that when nutrients (in this case carbon) is less, and light i restricted then growth is stopped or slowed down, growth of MED134 is shown to be in this case light and nutrient dependent and that at lower carbon levels PR has an influence on growth but at high levels of nutrients (C) it was less influential to the process.  

Further experiments were conducted such as transcription analysis which yielded some interesting results, where by the greater the incubation period the greater the tmRNA (kimura et al 2011) this paper suggests that it may be due to the low carbon conditions that were used as a variable. PR & retinal biosynthetic enzymes were also tested using the technique reverse transcriptase-PCR, where the RNA strand is reversed transcribed and the cDNA is amplified, this revealed that the PR gene will once again be dominant in light conditions rather than in a dark environment. Previous studies done by Lemi et al (2009) yielded similar results.

Further studies were conducted during this experiment from ATP synthesis, light sensors and also metabolic pathways. One final experiment  i’d like to draw your attention to is that of the transcriptome analysis   which found that in the light there was up-regulation of Sodium translocating NQR as well as cytrochrome c oxidase. Cytochrome c oxidase is the last respiratory enzyme in the ETC and allows for the production of ATP. This test shows the importance of the sodium ion gradient and the critical role of Na+ translocating NQR on the growth of the flavobacteria, (see paper for illustrations).

On the whole this study shows that in carbon limited environments, MED134 flavorbacteria requires PR utilisation in order to produce ATP and in turn grow, but only appears to be effective in light conditions and shows next to no response or use in dark environments.

If anyone is interested in reading the study in full please see the link below.   

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.