A method for enzymic extraction and the measurement of chloroplast RNA

Further characterization of ribosome binding to thylakoid membranes

Previous work indicated more polysomes bound to pea (Pisum sativum cv Progress No. 9) thylakoids in light than in the dark, in vivo (LE Fish, AT Jagendorf 1982 Plant Physiol 69: 814-825). With isolated intact chloroplasts incubated in darkness, addition of MgATP had no effect but 24 to 74% more RNA was thylakoid-bound at pH 8.3 than at pH 7. Thus, the major effect of light on ribosome-binding in vivo may be due to higher stroma pH. In isolated pea chloroplasts, initiation inhibitors (pactamycin and kanamycin) decreased the extent of RNA binding, and elongation inhibitors (lincomycin and streptomycin) increased it. Thus, cycling of ribosomes is controlled by translation, initiation, and termination. Bound RNA accounted for 19 to 24% of the total chloroplast RNA and the incorporation of [(3)H]leucine into thylakoids was proportional to the amount of this bound RNA. These data support the concept that stroma ribosomes are recruited into thylakoid polysomes, which are active in synthesizing thylakoid proteins.

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.

Studies on Genetic Male-Sterile Soybeans : III. The Initiation of Monocarpic Senescence

Soybean (Glycine max [L.] Merr.) germplasm, isogenic except for loci controlling male-sterility (ms(1)) and nodulation (rj(1)) was utilized to investigate the effects of reproductive tissue development and nitrogen source on the initiation of monocarpic senescence. The experimental genotypes (Ms(1)Rj(1), Ms(1)rj(1), ms(1)Rj(1), and ms(1)rj(1), were selected from a cross between N69-2774 and N59-5259, and were inbred to the F(5) generation. Green-house-grown plants were collected during the period of flowering (77 days after transplanting) until maturity (147 days after transplanting). Leaf tissues from the respective genotypes were analyzed at the various harvest dates for RNA, phenolic, and chlorophyll concentrations; acid protease activity; polypeptide banding patterns of chloroplast thylakoids; and chloroplastic ultrastructure.Regardless of nitrogen source, total chlorophyll concentrations declined between 77 and 119 days after transplanting, resulting in a 40% loss of chlorophyll per square centimeter in all genotypes. Leaf chlorophyll levels continued to decline at a constant rate in male-fertile genotypes, but remained at a constant level (26 micrograms chlorophyll per square centimeter) in male-sterile genotypes, for the remainder of the study. With increased leaf age, a gradual disruption of thylakoid structures was observed, particularly in chloroplasts from the male-fertile genotypes. Chloroplasts from the male-sterile genotypes appeared to lose starch grains but increased their number of chloroplastic lipid bodies with leaf aging. These data suggest that monocarpic senescence in soybeans was initiated at or before flowering. Although reproductive tissue development probably augmented the process, the response attributed to seed formation was not apparent until the mid-pod fill stage (119 days after transplanting). All genotypes had similar changes in other cellular components that are recognized as indicators of plant senescence regardless of whether the plants produced seed.

Optimal conditions for translation by thylakoid-bound polysomes from pea chloroplasts

Polysomes bound to washed thylakoids from pea Pisum sativum cv Progress No. 9 chloroplasts are capable of protein synthesis when supplemented with amino acids, ATP and a regenerating system, GTP, and soluble factors required for translation. The extent of protein synthesis in previous reports, however, was quite low when compared to in organello translation. By systematic testing of parameters in the isolation of thylakoids and reaction mixture components we have been able to establish more optimal conditions. Incorporation of 2 to 10 nanomoles of leucine per milligram chlorophyll in a 20-minute reaction period is now possible, representing a 10- to 60-fold increase over amounts previously reported. Autoradiographs of solubilized, electrophoresed membranes show about 30 discrete labeled polypeptides which remain associated with the thylakoid membranes.

Light-induced increase in the number and activity of ribosomes bound to pea chloroplast thylakoids in vivo

Within 8 to 10 minutes of illumination, chloroplast thylakoids of pea (Pisum sativum) became enriched 30 to 100% in ribosomes bound by nascent chains. Following (or, in some experiments, coincident with) this apprarent redistribution was a 25 to 65% increase in the total bound ribosome population, which was then maintained at this higher level during the normal light period. On transfer of plants to darkness, the bound ribosome population decreased to the lower dark level. White, blue (400 to 520 nanometers), and orange (545 to 690 nanometers) light were all effective in producing an increase in the bound ribosome population. The level of bound ribosomes in the oldest leaves of 16-day-old plants was 15-fold less than in the still-maturing leaf but was still increased by illumination.In vivo experiments with chloramphenicol and lincomycin indicated a requirement for protein synthesis by the 70S ribosomes both for the light-induced shift to the population bound by nascent chains and for the increase in the total thylakoid-bound population. When thylakoids from plants in darkness or exposed to light for increasing periods were incubated in an Eschericia coli cell-free protein synthesizing system, 15 minutes of prior illumination in vivo produced a 60% increase in [(3)H]leucine incorporation. This stimulation preceded the increase in total bound ribosomes but corresponded in time to observed increases in the ribosomes bound by nascent chains.A light intensity of 100 micromoles per meter(2) per second, but not 25 micromoles per meter(2) per second, caused a significant increase in bound ribosomes over a 30-minute period. Strong inhibition in vivo by 3',4'-dichlorophenyl-1, 1-dimethylurea suggests that noncyclic electron flow is essential for light-induced ribosome redistribution.

Bound Ribosomes of Pea Chloroplast Thylakoid Membranes: Location and Release in Vitro by High Salt, Puromycin, and RNase

The mode of attachment of 70S ribosomes to thylakoid membranes from pea leaves was studied by determining the proportion of the bound RNA which was released by various incubation conditions. The results supported a model in which several classes of bound ribosomes could be distinguished: (a) very tightly bound, not released by any conditions yet tested (20% of the total); (b) monomeric ribosomes attached by electrostatic interaction with the membranes (30 to 40% of the total) and released by high salt; and (c) polysomes, with some of the ribosomes attached by a combination of electrostatic interactions and insertion of the nascent polypeptide chain into the membrane. These required a combination of puromycin and high salt for release. Other ("hanging") ribosomes of the polysomes were inferred to be attached through mRNA but not actually attached to the membranes directly; they could be released by RNase under low salt conditions, as well as by puromycin plus high salt.To obtain these results, chloroplasts had to be prepared in media containing 0.2 molar Tris at pH 8.5. Using Tricine buffers at pH 7.5 yielded thylakoid membranes whose ribosomes were removed almost completely by high salt alone; these showed no response to puromycin. However, pH 7.5 had to be used in all cases for ribosome dissociation in high salt media, as the ribosome structure appeared to be degraded by high salt at pH 8.5, and release then occurred without the need for puromycin.The kinetics of ribosome release by high salt showed a rapid initial phase with a half-life of 20 seconds. The extent of release by high salt was very dependent on the temperature of the incubation. Plotting the data according to the Arrhenius interpretation shows a significant break at about 15 C, with apparent activation energy of 20 kilocalories per mole below that temperature and 5 kilocalories per mole above that temperature. This result suggests that membrane fluidity might be an important factor permitting release of ribosomes under high salt conditions.Electron microscope pictures of the washed thylakoids showed polysomes closely associated with the outer membranes of grana stacks, and with the stroma lamellae. Following digitonin treatment of the membranes and centrifugation, fractions enriched in Photosystem I and presumed stroma lamellae were also enriched in bound RNA.