An emerging molecular map of the phytochromes
Molecular mapping studies and sequence comparisons are providing provocative new insights into regions of the phytochrome polypeptide important to the functional activities of the photoreceptor. The NH 2 -terminal structural domain contains the determinants for photoperception, and for the differences in photosensory specificity and photolability between phyA and phyB. However, a contiguous COOH-terminal domain is also required for the transfer of perceived informational signals downstream to transduction pathway components and for PfrA-specific degradation to proceed. The COOH-terminal domains of phyA and phyB are functionally interchangeable in these processes and a core sequence at the proximal end of this domain contains determinants necessary for signal transfer from both phyA and phyB, suggesting a common biochemical mechanism of signal transfer for the two photoreceptors. Striking sequence similarity between the NH 2 -terminal domain of a Synechocystis protein, ORF SLR0473, and the phytochromes indicates that the cyanobacteria contain phytochrome-related photoreceptors. The COOH-terminal domains of ORF SLR0473 and the phytochromes are also related to one another and both show sequence similarities to the sensor histidine kinases. These data raise the possibility that the cyanobacteria have a functional photoregulated histidine kinase signalling system and that the plant phytochromes represent remnants of that system.