A MONOCLONAL ANTI-PHYTOCHROME ANTIBODY DETECTS A COMPLETELY UNRELATED PROTEIN IN CHLAMYDOMONAS-REINHARDTII

Screening of an expression cDNA library from Chlamydomonas reinhardtii with the monoclonal antibody Z-3B1 directed against phytochrome from Zea mays yielded the positive clone <<ZBI>>. The clone consists of 874 base pairs containing an open reading frame of 209 amino acids. The sequence has no homology with any phytochrome sequence but a remote homology with ubiH of Escherichia coli, which is supposed to be a flavoprotein (Nakahigashi et al., J. Bacteriol. 174, 7352-7359, 1992). Since the epitope for Z-3B1 previously mapped in oat phytochrome (Bonenberger et al., Photochem. Photobiol. 56, 717-723, 1992) is not contained in the sequence of ZBI, subclones of ZBI were prepared, expressed as fusion proteins and tested for reaction against the monoclonal antibody Z-3B1. By computer-aided folding of the peptide chain, a <<composed epitope>> was deduced that has sufficient homology with the authentic epitope in order to create cross-reactivity. Results obtained by heterologous screening with monoclonal antibodies have to be interpreted with caution.