THE PLANT CELL, Vol 1, Issue 3 361-371, Copyright © 1989 by American Society of Plant Biologists
Molecular and Biophysical Analysis of Herbicide-Resistant Mutants of Chlamydomonas reinhardtii: Structure-Function Relationship of the Photosystem II D1 Polypeptide
J. M. Erickson, K. Pfister, M. Rahire, R. K. Togasaki, L. Mets and J. D. Rochaix
Department of Biology, University of California, Los Angeles, California 90024
Plants and green algae can develop resistance to herbicides that block
photosynthesis by competing with quinones in binding to the chloroplast
photosystem II (PSII) D1 polypeptide. Because numerous herbicide-resistant
mutants of Chlamydomonas reinhardtii with different patterns of resistance
to such herbicides are readily isolated, this system provides a powerful
tool for examining the interactions of herbicides and endogenous quinones
with the photosynthetic membrane, and for studying the structure-function
relationship of the D1 protein with respect to PSII electron transfer. Here
we report the results of DNA sequence analysis of the D1 gene from four
mutants not previously characterized at the molecular level, the
correlation of changes in specific amino acid residues of the D1 protein
with levels of resistance to the herbicides atrizine, diuron, and bromacil,
and the kinetics of fluorescence decay for each mutant, which show that
changes at two different amino acid residues dramatically slow PSII
electron transfer. Our analyses, which identify a region of 57 amino acids
of the D1 polypeptide involved in herbicide binding and which define a D1
binding niche for the second quinone acceptor, QB of PSII, provide a strong
basis of support for structural and functional models of the PSII reaction
center.