Localization of protein-protein interactions between subunits of phytochrome

We have used a novel assay based on protein fusions with lambda repressor to identify two small regions within phytochrome's carboxy-terminal domain that are capable of mediating dimerization. Using an in vivo assay, fusions between the DNA binding, amino-terminal domain of lambda repressor and fragments from oat PhyA phytochrome have been assayed for increased repressor activity, an indicator of dimerization. In this assay system, regions of oat phytochrome between amino acids V623-S673 and N1049-Q1129 have been shown to increase repressor activity. These short spans are highly conserved between proteins belonging to the phytochrome PhyA family. Embedded within these sequences are four segments that could potentially form amphipathic alpha helices. Two of the segments are well conserved between PhyA phytochrome and phytochromes encoded by the phyB and phyC genes, suggesting that heterodimers might form by way of subunit interaction at these sites.