Light-harvesting complex assembly

Layered materials are used to produce supramolecular assemblies, in which the properties of the assemblies differ from those of the individual members of the assembly, such that the whole is more than the sum of its components. Such a behavior may be used as a criterion to define a supramolecular system, and nature provides numerous examples. The light-harvesting complex of the photosynthetic apparatus is one such example of outstanding organization. 

The present work illustrates in some detail the result of a unique mutation, one that impaired the biosynthesis of Chi b and resulted in a truncated Chi antenna size for the photosystems [Tanaka et al. 1998]. In the present Chi Mess mutant, PSII contained 93 Chi a molecules and PSI contained 246 Chi a molecules (Table 3). These antenna sizes are significantly larger than the PSII-core and PSl-core antennae, suggesting that Chi b may not be absolutely essential for the assembly of all Chi a-b light-harvesting complexes (see also [Ghirardi et al. 1986]). This was especially true for PSI which, in the Chi Mess mutant, had a Chi antenna size almost as large as that of the control (Table 3). It may be concluded that the Chi Mless mutation can be overcome by a nearly quantitative substitution of Chi b with Chi a in the Chi antenna of PSI, and by a partial substitution by Chi a in the antenna of PSII. 

Morrissey PJ, Glick RE and Melis A. (1989). Supramolecular assembly and function of subunits associated with the chlorophyll a-b light-harvesting complex II (LHCII) in soybean chloroplast. Plant Cell Physiol. 30, 335-344. 

Fig. 12. Simple models of various photosystems (A) purple bacteria, (B) green bacteria, (C) cyanobacteria and red algae, (D) bro n algae and (E) green plants. RC=reaction center FCPA=fucoxanthin-chlorophyll-protein assembly LHC=light-harvesting complex PS ll=photosystem II PS l=photosystem I. See text for discussion.

Fig. 1. Models for the light-harvesting complexes surrounding the PS-II reaction center in the thylakoid (A) Simplified model of the thylakoid in cross section, (B) More detailed model of PS II. (A) taken from Fig. 21 (A) in Chapter 1. The light-harvesting complex portion in (B) adapted from H-E Akerlund (1993) Function and organization of photosysfem II. In C Sundqvist and M Ryberg (eds) Pigment-Protein Complexes in Plastids Synthesis and Assembly, p 419. Acad Press.

I. Introduction Possible Structural Role of Carotenoids in the Assembly of Light-harvesting Complexes... 124... 

E. Assembly of Light-harvesting Complexes into Larger Units. 131... 

Chlamydomonas mutants that are entirely deficient in colored carotenoids do not accumulate reaction centers or light-harvesting complexes of Photosystem I or Photosystem II (Herrin et al., 1992). The accumulation of the mRNAs does not seem to be affected, so the failure in protein accumulation must be either due to inhibition of translation or rapid degradation of the proteins that cannot be assembled in the absence of carotenoids. Chi is also synthesized but very rapidly degraded in these mutants. 

Mutants of Scenedesmus obliquus have been desaibed that are carotenoid-deficient when grown in the dark because of a lack of phytoene synthase activity (Sandmaim et al, 1997). These mutants behave differently from the Chlamydomonas mutants in that they assemble a functional PS I but no PS II and no Chi a/b complexes (Humbeck et al., 1989 Romer et al., 1995). Upon illumination, carotenoids are formed and, concomitantly, PS II activity and light-harvesting complexes appear (Romer et al.,... 

Chlamydomonas mutants have been isolated that are deficient either in violaxanthin de-epoxidase or zeaxanthin epoxidase. Using these mutants, it could be demonstrated that only part of the non-photochemical quenching observed in Chlamydomonas is dependent on the formation of zeaxanthin (Niyogi etal., 1997a). It will be interesting to see whether the absence of xanthophyll epoxidase or de-epoxidase products has an effect on the assembly of light-harvesting complexes in these mutants. 

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