CHARACTERIZATION OF RECOMBINANT PHYTOCHROME FROM THE CYANOBACTERIUM SYNECHOCYSTIS

The complete sequence of the Synechocystis chromosome has revealed a p hytochrome-like sequence that yielded an authentic phytochrome when ov erexpressed in Escherichia coli, In this paper we describe this recomb inant Synechocystis phytochrome in more detail, Islands of strong simi larity to plant phytochromes were found throughout the cyanobacterial sequence whereas C-terminal homologies identify it as a likely sensory histidine kinase, a family to which plant phytochromes are related, A n approximate to 300 residue portion that is important for plant phyto chrome function is missing from the Synechocystis sequence, immediatel y in front of the putative kinase region, The recombinant apoprotein i s soluble and can easily be purified to homogeneity by affinity chroma tography, Phycocyanobilin and similar tetrapyrroles are covalently att ached within seconds, an antocatalytic process followed by slow confor mational changes culminating in red-absorbing phytochrome formation, S pectral absorbance characteristics are remarkably similar to those of plant phytochromes, although the conformation of the chromophore is li kely to be more helical in the Synechocystis phytochrome. According to size-exclusion chromatography the native recombinant apoproteins and holoproteins elute predominantly as 115- and 170-kDa species, respecti vely. Both tend to form dimers in vitro and aggregate under low salt c onditions, nevertheless, the purity and solubility of the recombinant gene product make it a most attractive model for molecular studies of phytochrome, including x-ray crystallography.