Release of ribosomes from thylakoids and endoplasmic reticulum with trypsin

Synthesis of two proteins in chloroplasts and mRNA distribution between thylakoids and stroma during the cell cycle of Chlamydomonas reinhardii

Chloroplasts contain thylakoid-bound and free ribosomes and polysomes. Whether binding of polysomes plays an immediate role in the regulation of chloroplast protein synthesis is not yet clear. In the present work, variations of protein synthesis and of mRNA content were measured not in greening, but in fully differentiated chloroplasts during the cell cycle of synchronized cultures of Chlamydomonas reinhardii. At different times of the vegetative cell cycle, the RNA was extracted from free and thylakoid-bound chloroplast polysomes and the partition of mRNAs between stroma and thylakoids was measured for two proteins, i.e. the 32-kDa herbicide-binding membrane protein and the soluble large subunit of the ribulose-1,5-bisphosphate carboxylase. At the same time the rates of synthesis of these two proteins were also determined. At 2 h after the onset of light, the content of both mRNAs in chloroplasts had doubled and 75-90% of each of these mRNAs were found to be bound to the thylakoids. The rate of protein synthesis, however, increased 10-fold, but reached its maximum only after about 6 h in the light. The differences in the time courses, in the stimulation of the rate of protein synthesis, and in the mRNA-binding to thylakoids point to a translational regulation of protein synthesis. Furthermore, since a very high proportion of polysomes were bound to thylakoids, containing mRNA for both a membrane and a soluble protein, this light-induced binding of polysomes to thylakoids seems to be an essential, but not the only, prerequisite for protein synthesis in chloroplasts.

Chloroplast messenger RNAs of free and thylakoid-bound polysomes from Vicia faba L

Purified chloroplasts from developing leaves of Vicia faba L. were broken and separated into stroma and thylakoid fractions. Both fractions contained polysomes as demonstrated by analytical density gradient centrifugation and in-vitro read-out translation. Messenger RNAs of free and thylakoid-bound polysomes were isolated and analysed by hybridization with heterologous gene probes from spinach and tobacco. Transcripts of the chloroplast genes psaA, psbB, psbC, psbD and petA were found predominantly on thylakoidbound polysomes engaged in the synthesis and the contrasslational integration of membrane proteins. In contrast, transcripts of the genes rbcL, psbE, petD, atpA, atpB, atpE and atpH were found more frequently on free polysomes corresponding to a stroma-located translation of these mRNAs and a posttranslational integration of the encoded intrinsic membrane proteins. We conclude from these findings that chloroplast-encoded membrane proteins are integrated by co-and posttranslational mechanisms.

Synthesis of large subunit of ribulosebisphosphate carboxylase by thylakoid-bound polyribosomes from spinach chloroplasts

Intact chloroplasts were isolated from developing first leaves of spinach. The chloroplasts were broken and separated into an extensively washed membrane (thylakoid) fraction and a soluble (stroma) fraction. The membrane fraction contained polyribosomes with properties similar to those of thylakoid-bound polyribosomes of other organisms. The distribution of mRNA for large-subunit ribulosebisphosphate carboxylase (LS) was determined by translating RNA from chloroplasts, thylakoids, and stroma in a wheat germ cell-free translation system. LS translation product was identified by immunoprecipitation with antibody to LS from spinach, electrophoresis of the immunoprecipitated product, and fluorography. At least 44% of translatable chloroplast LS-mRNA was in the washed thylakoid fraction. Thylakoid-bound LS-mRNA was in polyribosomes since LS was produced by thylakoids in an Escherichia coli cell-free translation system under conditions where initiation did not take place. Our results demonstrate that membrane-bound polyribosomes can synthesize the stroma-localized polypeptide LS, and suggest that the thylakoids may be an important site of its synthesis.

Thylakoid membranes: the translational site of chloroplast DNA-regulated thylakoid polypeptides

Stromal ribosomes and those bound to thylakoid membranes were prepared from intact spinach chloroplasts which were purified on Percoll gradients. The products of read-out translation of these ribosomes supplemented with an Escherichia coli extract were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Striking similarity was found between the polypeptides labeled in the read-out translation of the chloroplastic ribosomes and those synthesized in isolated chloroplasts. Among the polypeptides translated on thylakoid-bound ribosomes, apoprotein of chlorophyll-protein complex I, alpha and beta subunits of coupling factor 1, and 32,000-Da membrane polypeptide were identified from their mobility on the polyacrylamide gel. The large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase and other several stromal proteins were translated exclusively from stromal ribosomes. However, when the translation was programmed in cell-free systems from either E. coli, wheat germ, or rabbit reticulocytes by RNAs isolated separately from stroma and thylakoids, no qualitative difference was found between the products from those RNAs. These results suggest that thylakoid-bound ribosomes are the main sites of synthesis of thylakoid proteins and stromal-free ribosomes are that of stromal proteins, and that thylakoids and stroma contain mRNAs for the stromal and the thylakoid proteins, respectively, in a form not functioning in the chloroplasts.

Synthesis of photosynthetic membrane proteins directed by RNA from rough thylakoids of Chlamydomonas reinhardtii

Messenger RNA activities associated with thylakoids of Chlamydomonas reinhardtii were examined by translation of RNA of rough thylakoids (thylakoids with bound ribosomes) in a wheat germ protein-synthesis system. The RNA fraction, active in translation, did not bind to oligod (T)-cellulose indicating that the mRNA associated with the rough thylakoids contains little, if any, 3' polyadenylic acid. Assuming that 1% of the thylakoid RNA was mRNA, it was found to have a specific translation activity between that of globin mRNA and tobacco mosaic virus RNA. Translation products of thylakoid RNA were immunoprecipitated by crossed immunoelectrophoresis, using antisera to two polypeptides synthesized within the chloroplast, one of which specifically precipitates the beta subunit of chloroplast coupling factor 1. Each antiserum immunoprecipitated radioactive polypeptide from translates of thylakoid RNA, indicating that the presence of mRNA for both polypeptides is associated with the thylakoids. The results suggest that thylakoid-bound polyribosomes are involved in the synthesis of thylakoid polypeptides.

Sub-thylakoid fractions containing ribosomes

A sub-membrane fraction which contains a large portion of any thylakoid-bound ribosomes can be obtained when thylakoids are treated with the detergents Nonidet P-40, or Triton X-100. These 'pseudopolysome' fractions contain 50% of thylakoid-bound ribosomes, but less than 0.5% of thylakoid chlorophyll. Triton and Nonidet psuedopolysomes contain about 10%, and 3% of thylakoid protein, respectively. Pseudopolysomes, prepared from thylakoids with low levels of ribosomes, contain about the same proportion of thylakoid protein but proportionately less ribosomes. Pseudopolysomes contain thylakoid polypeptides in addition to chlorophyll, but lack a major membrane polypeptide of Mr 50 000. Pseudopolysome chlorophyll, and RNA band at the same buoyant density. However, they band at different densities after pseudopolysomes are treated with trypsin (a procedure which strips thylakoids of ribosomes). Pseudopolysome fractions from thylakoids with low levels of ribosomes have a lower density than the corresponding fractions from thylakoids with high levels of ribosomes. Ribosomes are released from thylakoids, and pseudopolysomes by the same treatments. Subunits are released with KCl and puromycin. Polysomes are released with trypsin. It was concluded the pseudopolysomes consist of ribosomes and a membrane fragment containing the sites to which ribosomes are bound.