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Kinetic Behavior of Electron Paramagnetic Resonance Signal I. II Comparison of Wild Type and Mutant (ac-206) Chlamydomonas reinhardtii¹

^a Exobiology Division, Ames Research Center, NASA, Moffett Field, California 94035

The electron paramagnetic resonance (EPR) characteristics of wild type Chlamydomonas reinhardtii are compared with those of a mutant strain (ac-206) which lacks cytochrome 553. The steady-state signals I and II are similar but differ in their responses to light of long and short wavelengths, reflecting the fact that the electron transport chain linking photosystems I and II is interrupted. The kinetic behavior of signal I is simpler in the mutant, which lacks induction effects prominent in the wild type. The decay of the signal when light ceases is not dependent on the length or intensity of illumination in the mutant, whereas it is in the wild type. These data can be interpreted in terms of signal I being a reflection of cyclic flow in a pathway which does not involve cytochrome 553 in the mutant, whereas in the wild type there is also a contribution of electrons from photosystem II.