Genetic engineering of phytochrome biosynthesis in bacteria.

The bilin prosthetic groups of the phytochrome photoreceptors and the light-harvesting phycobiliprotein antennae arise from the oxygen-dependent ring opening of heme. Two ferredoxin-dependent enzymes contribute to this conversion: a heme oxygenase and a bilin reductase with discrete double-bond specificity. Using a dual plasmid system, one expressing a truncated cyanobacterial apophytochrome 1, Cph1(N514), and the other expressing a two-gene operon consisting of a heme oxygenase and a bilin reductase, these studies establish the feasibility of producing photoactive phytochromes in any heme-containing cell. Heterologous expression systems for phytochromes not only will facilitate genetic analysis of their assembly, spectrophotometric activity, and biological function, but also might afford the means to regulate gene expression by light in nonplant cells.