THYLAKOID MEMBRANE-DEVELOPMENT AND DIFFERENTIATION - ASSEMBLY OF THE CHLOROPHYLL A-B LIGHT-HARVESTING COMPLEX AND EVIDENCE FOR THE ORIGIN OF M(R)=19, 17.5 AND 13.4 KDA PROTEINS

Pea plants were grown under intermittent illumination (ImL) conditions . The low dosage of light given to ImL plastids limited the rate of ch lorophyll (Chl) a and Chl b biosynthesis and, therefore, it retarded t he rate of photosynthetic unit formation and thylakoid membrane develo pment. Depending on the developmental stage of the photosynthetic unit , ImL plastids had variable Chl a/Chl b ratios (2.7<Chl a/Chl b<20) an d showed distinct intermediates in the assembly of the chlorophyll a-b light-harvesting complex (LHC) of photosystem-II (PSII). The results are consistent with a step-wise increment in the PSII antenna size inv olving three distinct forms of the PSII unit: (i) a PSII-core form wit h about 37 Chl a molecules; (ii) a PSIIbeta form containing the PSII-c ore and the LHC-II-inner antenna with a total of about 130 Chl (a+b) m olecules, and (iii) the mature PSIIa form containing PSIIbeta and the LHC-II-peripheral antenna with a total of 210-300 Chl (a+b) molecules. The thylakoid membrane contained polypeptide subunits b, c and d (the Lhcb1, 2 and 3 gene products, respectively) when only the LHC-II-inne r was present. Polypeptide subunit a, (the apoprotein of the chlorophy ll-protein known as CP29), along with increased amounts of b and c app eared later in the development of thylakoids, concomitant with the ass embly of the LHC-II-peripheral. The results suggest that polypeptide s ubunit d has priority of assembly over subunit a. It is implied that, of all LHC-II constituent proteins, subunit d is most proximal to the PSII-core complex and that it serves as a linker in the transfer of ex citation energy from the bulk LHC-II (subunits b and c) to the PSII-co re. The work also addresses the origin of low-molecular-weight protein s (M(r)=19, 17.5 and 13.4 kDa) which coisolate with intact developing plastids and whose abundance decreases during plastid development. Ami no acid compositional and immunoblot analyses show a nuclear histone o rigin for these low-molecular-weight proteins and suggest co-isolation of histone-containing nuclear vesicles along with intact developing p lastids.