The PSI-K subunit of photosystem I is involved in the interaction between light-harvesting complex I and the photosystem I reaction center core

PSI-K is a subunit of photosystem I. The function of PSI-K was characterize d in Arabidopsis plants transformed with a psaK cDNA in antisense orientati on, and several lines without detectable PSI-K protein were identified. Pla nts without PSI-K have a 19% higher chlorophyll alb ratio and 19% more P700 than wild-type plants. Thus, plants without PSI-R compensate by making mor e photosystem I. The photosystem I electron transport in vitro is unaffecte d in the absence of PSI-K. Light response curves for oxygen evolution indic ated that the photosynthetic machinery of PSI-K-deficient plants have less capacity to utilize light energy. Plants without PSI-K have less state 1-st ate 2 transition. Thus, the redistribution of absorbed excitation energy be tween the two photosystems is reduced. Low temperature fluorescence emissio n spectra revealed a 2-nm blue shift in the long wavelength emission in pla nts lacking PSI-K. Furthermore, thylakoids and isolated PSI without PSI-K h ad 20-30% less Lhca2 and 30-40% less Lhca3, whereas Lhca1 and Lhca4 were un affected. During electrophoresis under mildly denaturing conditions, all fo ur Lhca subunits were partially dissociated from photosystem I lacking PSI- K, The observed effects demonstrate that PSI-K has a role in organizing the peripheral light-harvesting complexes on the core antenna of photosystem I .