THE AMINO-TERMINUS OF PHYTOCHROME-A CONTAINS 2 DISTINCT FUNCTIONAL DOMAINS

The N-terminus of phytochrome A is important for the structural integr ity and biological activity of the photoreceptor. Mutational analysis of the N-terminus by two different strategies created two distinct pho toreceptors, one inactive and the other hyperactive when expressed in transgenic tobacco, suggesting that this region has multiple functiona l domains. To identify critical residues within this N-terminal region , a series of smaller deletions of oat phytochrome A were created, des ignated NE (Delta 49-62), NC (Delta 6-47), ND (Delta 7-21), NE (Delta 2-5), and NF (Delta 6-12), and compared with a previously characterize d deletion mutant NA (Delta 7-69) and full-length oat phytochrome A. U sing photochemical properties as a measure of chromoprotein structure, it was found that the region between residues 13 and 62 was important for the spectral integrity of the photoreceptor. These deletion mutan ts were also biologically inactive when expressed in both mature tobac co plants and seedlings grown under continuous far-red or red light. I n contrast, deletion of the serine-rich region between residues 6 and 12 did not alter the photochemical properties but did produce a hypera ctive photoreceptor, indicating this region may be involved in down-re gulating phytochrome A activity. The data show that the N-terminus of phytochrome A contains two functional domains, one necessary for confo rmational stability and biological activity (residues 13-62), and the other involved in attenuating phytochrome responses (residues 6-12).