Identification And Functional Study Of Photoreceptors In Phototactic Cyanobacterium Synechococcus Elongatus UTEX 3055

Cyanobacteria is an ancient procaryote that performs oxygenic photosynthesis.As in plant,exposure to proper sunlight is important for cyanobacterial cell growth.Over hundreds of millions of years of evolution,many cyanobacteria have evolved the ability to actively seek out more suitable light conditions,a process called phototaxis.Cyanobacterial phototaxis behavior and the underlying molecular mechanisms have been extensively studied in a round-shaped model cyanobacteria Synechocystis sp.PCC 6803.Studies from the model organism and sequence analysis have revealed that cyanobacteria possess many cyanobacteriochromes(CBCRs),which are GAFcontaining photoreceptors.CBCR with a wide range of spectrum ranging from ultraviolet light to far-red light have been reported.Synechococcus elongatus UTEX3055 is a newly isolated,unicellular short rod-shaped cyanobacteria,which has been shown to have good phototaxis.In this study,we systematically investigated the response spectrum of UTEX 3055 phototactis by detecting its phototactic distance under a single wavelength of light.Our results showed that UTEX 3055 exhibited slightly negative phototaxis when exposed to short wavelength light at 405 nm and 450 nm,but no phototaxis motility was observed at any other wavelengths tested.In addition,to identify potential photoreceptors in UTEX 3055,we applied bioinformatics technology to perform sequence alignment analysis on the whole genome of UTEX3055 and found three potential photoreceptors.Further protein purification,zinc staining(Zinc-blot)and absorption spectrum analysis results showed that UTEX3055＿2687 protein has the ability to bind cyanobacteriochrome,and has obvious absorption peaks in the blue(425 nm)and orange(600 nm)wavelength,which proves that UTEX 3055＿2687 is a CBCR that senses blue/orange light.Further experiments to monitor the absorption peak transition under dark conditions showed that the dark stable state of this photoacceptor is a blue light absorption state(425 nm).In order to study the function of UTEX 3055＿2687 in cells,we constructed deletion mutants and overexpressed strains of this gene,and tested the phototaxis ability of the mutant strains.The results showed that compared with WT,the phototaxis of the deletion mutant was weakened and the colony morphology was significantly changed,while the overexpression mutant had enhanced phototaxis.At the same time,we measured the growth curve of WT,UTEX 3055＿2687 deletion mutant and UTEX 3055＿2687 overexpression mutants.The results showed that the deletion or overexpression of this gene did not affect the growth of the strain.In the phenotype screening,we also found that the protein encoded by UTEX 3055＿2687 has no interaction with circadian clock-related genes,indicating that the photoreceptor may not be involved in the regulation of the circadian clock of UTEX 3055.This study helps us better understand how cyanobacteria sense ambient light signals through photoreceptors and provides potential CBCR for optogenetic research and applications.