Changes in Photosystem II during Biosynthesis of Components and Development of Function in Chloroplasts of a Greening Mutant of Scenedesmus

Dark-grown cells of the mutant C-2A' of Scenedesmus obliquus,which lack chlorophyll and photosynthetic activities, developa fully functional photosynthetic apparatus after transfer tolight (Bishop and Senger. 1972, Senger and Bishop 1972). Afteronset of illumination PS II-activity increases rapidly. Simultaneouslythe apoproteins of the two PS II chlorophyll -protein complexesCP-a_11–1 and CP-a_11–2 (48 and 44 kDa) are formedat high rates, as shown by fluorography after ³⁵S-label duringdifferent periods of development. Polypeptides with apparentmolecular weights of 32.5 (probably the manganese-binding polypeptideof the oxygen-evolving system), 19.5, 18, 17and 16.5 kDa aresynthesized with kinetics comparable to those of the 48 and44 kDa polypeptides. Whereas the apoproteins of CP-a_11–1and CP-_11–2 are already present in etioplasts and areheavily formed immediately after onset of illumination, thepolypeptides related to the light-harvesting complex CP-a/bcannot be detected in dark-grown cells and show high rates ofbiosynthesis only after a delay of about 1 hour. An asynchronousfashion of formation is also reported for the correspondingchlorophyll-protein complexes of PS II. Our findings prove astep-wise assembly of PS II during chloroplast development inC-2A', starting with small PS II-units composed of the core-complexes,which increase their amount of light-harvesting complexes duringfurther illumination. High values for PS II-activity/chlorophylland for the half-rise time of fluorescence-induction in earlystages of greening, which decrease rapidly during prolongedillumination, also indicate the change from a small to a largePS Il-unit. Furthermore, investigation of the formation of thylakoidmembrane polypeptides under the influence of different protein-biosynthesisinhibitors of 70 S- or 80 S-ribosomes by means of ³⁵S-labeland subsequent fluorography revealed that most of these polypeptidesare coded by nuclear genes. Only bands at 68, 65.5, 53, 52,48, 44, 32.5, 16.5, 15 and 14.5 kDa were labelled in the presenceof 80 S-inhibitors indicating their chloroplast origin. ¹Present address: Fachbereich Biologie-Botanik, Philipps-Universit?t,I.ahnberge, 3550 Marburg, Federal Republic of Germany. (Received April 14, 1986; Accepted August 13, 1986)