Proteorhodopsin photosystem gene expression enables photophosphorylation in a heterologous host.
Proteorhodopsins (PRs) are retinal-containing proteins that catalyze light-activated proton efflux across the cell membrane. These photoproteins are known to be globally distributed in the ocean's photic zone, and they are found in a diverse array of Bacteria and Archaea. Recently, light-enhanced growth rates and yields have been reported in at least one PR-containing marine bacterium, but the physiological basis of light-activated growth stimulation has not yet been determined. To describe more fully PR photosystem genetics and biochemistry, we functionally surveyed a marine picoplankton large-insert genomic library for recombinant clones expressing PR photosystems in vivo. Our screening approach exploited transient increases in vector copy number that significantly enhanced the sensitivity of phenotypic detection. Two genetically distinct recombinants, initially identified by their orange pigmentation, expressed a small cluster of genes encoding a complete PR-based photosystem. Genetic and biochemical analyses of transposon mutants verified the function of gene products in the photopigment and opsin biosynthetic pathways. Heterologous expression of six genes, five encoding photopigment biosynthetic proteins and one encoding a PR, generated a fully functional PR photosystem that enabled photophosphorylation in recombinant Escherichia coli cells exposed to light. Our results demonstrate that a single genetic event can result in the acquisition of phototrophic capabilities in an otherwise chemoorganotrophic microorganism, and they explain in part the ubiquity of PR photosystems among diverse microbial taxa.
Martinez
A
A
Department of Civil and Environmental Engineering, Division of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Bradley
A S
AS
Waldbauer
J R
JR
Summons
R E
RE
DeLong
E F
EF
D000255:Adenosine Triphosphate / Q000737:chemistry; D019843:Archaeal Proteins / Q000737:chemistry; D001426:Bacterial Proteins / Q000737:chemistry; D002462:Cell Membrane / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D015723:Gene Library; D022761:Gene Transfer, Horizontal; D008027:Light; D008956:Models, Chemical; D008957:Models, Genetic; D008969:Molecular Sequence Data; D010766:Phosphorylation; D045322:Photosynthetic Reaction Center Complex Proteins; D012243:Rhodopsin / Q000235:genetics*; D025604:Rhodopsins, Microbial
D019843:Archaeal Proteins; D001426:Bacterial Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D025604:Rhodopsins, Microbial; C413548:proteorhodopsin; D000255:Adenosine Triphosphate; D012243:Rhodopsin