Chlamydomonas, carotenoid-deficient mutant

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 seems to be more similar to Scenedesmus than to Arabidopsis with respect to the consequences of j3-e carotenoid deficiency. In the lutl mutant, an increased Chi a b ratio indicates a partial defect in the assembly or stability of Chi a/b complexes (Eichenberger et al, 1986). However, at least some of the LHCII subunits are still assembled. Since these complexes are thought to contain lutein in the wildtype, this means that /S-carotene-type xanthophylls are probably able to replace lutein during the assembly and in stabilizing these complexes (Niyogi et al., 1997b). 

Eichenberger W, Boschetti A and Michel HP (1986) Lipid and pigment composition of a chlorophyll h-deficient mutant of Chlamydomonas reinhardttii. Physiol Plant 66 589-594 Ermler U, Fritzsch G, Buchanan SK and Michel H (1994) Structure of the photosynthetic reaction centre from Rhodobacter sphaeroides at 2,65 A resolution Cofactors and protein-cofactor interactions. Structure 2 925-936 Foidl M, Golecki JR and Oelze J (1997) Phototrophic growth and chlorosome formation in Chloroflexus aurantiacus under conditions of carotenoid deficiency. Photosynth Res 54 219-226... 

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. 

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