Iron stress restricts photosynthetic intersystem electron transport in Synechococcus sp, PCC 7942

Although exposure of Synechococcus sp. PCC 7942 to iron stress induced the accumulation of the isiA gene product (CP43') compared with non-stressed co ntrols, immunodetection of the N-terminus of cytochrome (Cyt) f indicated t hat iron stress not only reduced the content of the 40 kDa, heme-binding, C yt f polypeptide by 32% but it also specifically induced the accumulation o f a new, 23 kDa, non-heme-binding, putative Cyt f polypeptide, Concomitantl y, iron stress restricted intersystem electron transport based on the in vi vo reduction of P700(+), monitored as DeltaA(820)/A(820) in the presence an d absence of electron transport inhibitors, as well as the inhibition of th e Emerson enhancement effect on O-2 evolution. However, iron stress appeare d to be associated with enhanced rates of PS I cyclic electron transport, l ow rates of PS I-driven photoreduction of NADP(+) but comparable rates for PS II+PS I photoreduction of NADP(+) relative to controls. We hypothesize t hat Synechococcus sp, PCC 7942 exhibits a dynamic capacity to uncouple PS I I and PS I electron transport, which may allow for the higher than expected growth rates observed during iron stress. (C) 2000 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserv ed.