Biochimica et Biophysica Acta, 1060 (1991) 59-66 © 1991 Elsevier Science Publishers B.V. All rights reserved 0005-2728/91/$03.50 ADONIS 0005272891002212 59 BBABIO 43475 Structure and energy transfer of the phycobilisome in a linker protein replacement mutant of cyanobacterium Synechococcus 7942 Rishikesh P. Bhalerao 1, Tomas Gillbro 2 and Petter Gustafsson 1 1 Department of Plant Physiology, Ume~ University, Ume~ (Sweden) and 2 Department of Physical Chemistry, Ume~t University, Ume?~ (Sweden) (Received 12 February 1991) (Revised manuscript received 8 May 1991) Key words: Phycobilisome;Energy transfer; Photosynthesis; (Cyanobacterium); (Synechococcus 7942) The role of the linker proteins in the biogenesis and energy transfer of the phycobilisome rod was monitored by making insertional inactivation in the cpcl gene coding for the core-proximal 33 kilodalton (kDa) protein in the cyanobacterium Synechococcus 7942. The insertion leaves the cpcH gene coding for the core-distal 30 kDa protein intact and functional. Analysis of the phycobilisome protein composition of the cpcl mutant shows that the 30 kl)a protein is present in normal amounts in the rod, indicating that the 30 kDa linker protein can replace the 33 kDa protein in the biogenesis and structural integrity of the rod. The absorption and fluorescence characteristics of the mutated phycobilisome is almost indistinguishable from that of the wild-type of the same rod length. The fluorescence kinetics from the cpcl mutant show that the dominating decay component has a lifetime from phycocyanin of 69 ps as compared to 72 ps found for the wild-type phycobilisome with the same rod length. The results show that replacing the 33 kDa for the 30 kDa linker in the rod does not alter the energy harvesting or the energy transfer characteristics of the rod in contrast to what has been concluded from data obtained from in vitro experiments. We conclude that the linker polypeptides have only a minor influence on the energy transfer characteristics of the rod but are mainly involved in determining the length of the rod in response to changing environmental light conditions. Introduction The phycobilisome serves as the light harvesting antennae in cyanobacteria [1-4]. It harvests photons and transfers the energy to the photosynthetic reaction centre of Photosystem II [1-4]. The phycobilisome, which looks like a hemispheric disc in the electronmi- croscope, is a multi-protein complex with a highly ordered structure consisting both of chromophorylated polypeptides as well as non-chromophorylated struc- tural linker proteins [2-4]. It is built up as a core component attached to the thylakoid membrane with rods bound to the core, the rods pointing outwards [4]. Abbreviations: PBS, phycobilisome; C-PC, cyanobacterial phyco- cyanin; APC, allophycocyanin;kb, kilobase pairs; kDa, kiloDalton. Correspondence: P. Gustafsson, Department of Plant Physiology, Ume~ University, S-901 87 Ume~t, Sweden. The chromophoric proteins in the rod have their light absorption maxima at a shorter wavelength than the chromophoric proteins in the core thereby focusing the excitation energy efficiently down to the chlorophyll molecules in the reaction centre. The phycobilisomes of the cyanobacterium Syne- chococcus 6301/7942 (also denoted Anacystis nidulans 625/R2) has an over-all absorption maximum at ap- prox. 625 nm [5-7]. It contains the chromophoric pro- teins allophycocyanin (APC) and phycocyanin (C-PC) in its core and rod, respectively. Each protein consists of two subunits, a and /3. The rod length can be changed in a highly regular manner in accordance with changing environmental conditions [8,9]. In its fully extended form it contains three discs, each of which is a double trimer of phycocyanin, (0:3/33)X 2 [5]. Four non-chromophorylated linker polypeptides are posi- tioned at specific places in the rod, thus it is more complex than the rods found in the cyanobacterium Agmenellum quadruplicatum turn [10]. A linker protein