Posttranslational modification of oat phytochrome A: Phosphorylation of a specific serine in a multiple serine cluster

Phytochrome A (phyA) is a photoreceptor of higher plants which mediates a variety of biochemical and physiological processes in response to red/far-red light. By detailed structural analysis of the peptides of the total tryptic digest of oat phyA, we found that the photoreceptor isolated from red light irradiated seedlings contains only one site of phosphate attachment, in the N-terminal Ser-rich region. The N-terminal tryptic phosphopeptide (residues 1-12) contains eight serine residues, any of which may be phosphorylated. Direct fast atom bombardment mass spectrometry (FAB MS/MS) analysis of the phosphorylated peptide as well as of its phosphate-containing fragment (residues 1-9) was not successful due to their hydrophilic nature and instability of the phosphate bond. beta-Elimination of the phosphorylated tryptic peptide in the presence of ethanethiol converted the phosphoserine residue to S-ethylcysteine that is stable under FAB MS/MS. FAB MS/MS analysis of the modified peptide clearly showed that the phosphate group was attached to Ser(7). The in vivo phosphorylation site at Ser(7) in oat phyA is discussed for its possible regulatory role in phyA function.