Influence of thylakoid protein phosphorylation on Emerson enhancement and the quantum requirement of Photosystem I

Abstract

The relative size of the antennae of PS I and PS II has been estimated from 420 to 722 nm in isolated granal thylakoids, by measuring electron transport from H 2O to NADP upon flash actinic illumination under conditions of linearity of electron transport versus light intensity. Emerson enhancement of NADP reduction by PS I-absorbed radiation of 722 nm had a large peak at 475–500 nm, dropping to zero at 520 nm, and a minor one at 650 nm. No enhancement was observed from 520 to 640 nm, nor at wavelengths above 670 nm. Phosphorylation of LHC-II suppressed the enhancement at all wavelengths: upon phosphorylation, the antenna of PS I became larger than that of PS II over the entire spectrum, including the 475–500 nm region. The quantum requirement of PS I was measured from 420 to 691 nm in the granal thylakoids. A minimum was observed at 520 nm, and maxima at 475 to 500 and 650 to 670 nm, i.e., corresponding to the LHCII maximal absorption.