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Journal of Bacteriology, June 2002, p. 2889-2897, Vol. 184, No. 11
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.11.2889-2897.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Identification of a Lycopene ß-Cyclase Required for Bacteriorhodopsin Biogenesis in the Archaeon Halobacterium salinarum

Ronald F. Peck,^1, Eric A. Johnson,² and Mark P. Krebs^1*

Department of Biomolecular Chemistry, University of Wisconsin Medical School,¹ Department of Food Microbiology and Toxicology and Department of Bacteriology, University of Wisconsin, Madison, Wisconsin 53706²

Received 10 January 2002/ Accepted 8 March 2002

Biogenesis of the light-driven proton pump bacteriorhodopsin in the archaeon Halobacterium salinarum requires coordinate synthesis of the bacterioopsin apoprotein and carotenoid precursors of retinal, which serves as a covalently bound cofactor. As a step towards elucidating the mechanism and regulation of carotenoid metabolism during bacteriorhodopsin biogenesis, we have identified an H. salinarum gene required for conversion of lycopene to ß-carotene, a retinal precursor. The gene, designated crtY, is predicted to encode an integral membrane protein homologous to lycopene ß-cyclases identified in bacteria and fungi. To test crtY function, we constructed H. salinarum strains with in-frame deletions in the gene. In the deletion strains, bacteriorhodopsin, retinal, and ß-carotene were undetectable, whereas lycopene accumulated to high levels (1.3 nmol/mg of total cell protein). Heterologous expression of H. salinarum crtY in a lycopene-producing Escherichia coli strain resulted in ß-carotene production. These results indicate that H. salinarum crtY encodes a functional lycopene ß-cyclase required for bacteriorhodopsin biogenesis. Comparative sequence analysis yields a topological model of the protein and provides a plausible evolutionary connection between heterodimeric lycopene cyclases in bacteria and bifunctional lycopene cyclase-phytoene synthases in fungi.

Present address: Department of Medical Microbiology and Immunology, University of Wisconsin Medical School, Madison, WI 53706.

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