Bacterium Lacking a Known Gene for Retinal Biosynthesis Constructs Functional Rhodopsins

Area of Science:

• Microbiology
• Biochemistry
• Molecular Biology

Background:

• Microbial rhodopsins are light-driven proteins essential for various cellular functions.
• Some prokaryotes possess rhodopsins but lack known genes for retinal biosynthesis, the light-absorbing component.
• This raises questions about the functionality of rhodopsins in these organisms.

Purpose of the Study:

• To investigate the presence and function of rhodopsin in Aurantimicrobium minutum KNC^T, a terrestrial bacterium lacking known retinal biosynthesis genes.
• To determine if A. minutum KNC^T can produce functional rhodopsins without external retinal.
• To explore the implications for rhodopsin functionality in prokaryotes with incomplete retinal pathways.

Main Methods:

• Measuring ion transport activity via light-induced pH changes in cultured A. minutum KNC^T cells.
• Spectroscopic analysis of cell lysates to detect rhodopsin components.
• High-Performance Liquid Chromatography coupled with tandem Mass Spectrometry (HPLC-MS/MS) to identify retinal.

Main Results:

• A light-induced pH change was observed in A. minutum KNC^T cell suspensions, indicating ion transport activity.
• Endogenous retinal was detected in the cell lysate, confirming its presence within the bacteria.
• Spectroscopic and HPLC-MS/MS analyses confirmed the presence of functional rhodopsin in A. minutum KNC^T.

Conclusions:

• Aurantimicrobium minutum KNC^T possesses functional rhodopsin and produces its own retinal through an unidentified biosynthetic pathway.
• The findings challenge the hypothesis that rhodopsins are non-functional in prokaryotes lacking known retinal biosynthesis genes.
• It suggests that many rhodopsin-possessing prokaryotes may have functional rhodopsins despite lacking canonical retinal production genes.