Amino acid incorporation and products of protein synthesis in isolated chloroplasts of Euglena gracilis

Protein synthesis by isolated Acetabularia chloroplasts. In vitro synthesis of the apoprotein of the P-700-chlorophyll alpha-protein complex (CP i)

Acetabularia chloroplasts can incorporate radioactive amino acids for up to several hours in vitro. The incorporation is sensitive to chloramphenicol and lincomycin, insensitive to cycloheximide, and completely light-dependent. At least 35 discrete labelled bands can be separated by SDS-polyacrylamide gel electrophoresis: 20--24 in the soluble fraction and 13--15 in the membrane fraction. Most of the label (80--85%) is in the membrane fraction, and 90% of that is in a polypeptide of 32 000 daltons. Chlorophyll-protein complexes were purified from in vitro labelled chloroplasts by SDS-polyacrylamide gel electrophoresis. CP I (P-700-chlorophyll alpha-protein complex) and its apoprotein were both labelled. This shows that the apoprotein is synthesized on chloroplast ribosomes, and can be integrated correctly into the thylakoid membrane in the absence of any cytoplasmic contribution. In contrast, no label was incorporated into the two polypeptides of CP II, the light-harvesting chlorophyll a/b complex.

Counting the genes for stabel RNA in the nucleus and chloroplasts of Euglena

Improved procedures for counting the genes for ribosomal and 4 S RNA in the nucleus and chloroplasts of Euglena are described. These procedures exploit (a) the chloroplast-free strain ZHB, (b) the flotation method for purifying chloroplasts, (c) Hg2+/Cs2SO4 density gradients to fractionate large quantities of DNA and (d) more efficient conditions of DNA - RNA hybridization particularly for 4 S RNA. We find that the average Euglena cell in an exponentially growing culture contains 800 nuclear genes for each cytoplasmic rRNA and 800-880 chloroplast genes for chloroplast rRNA. Each chloroplast chromosome of molecular weight 92 - 10(6) contains two genes for each rRNA. The number of nuclear genes for 4 S RNA is approx. 760. The total number of chloroplast genes for 4S RNA is approx. 10 000, or 22-25 on each chloroplast chromosome.

Ferredoxin biosynthesis in Euglena gracilis

Analysis of ferredoxin content in cultures of Euglena gracilis grown in the presence of selective antibiotic inhibitors of protein synthesis resulted in the following conclusions: 1. Ferredoxin is synthesized from cytoplasmic (80s-type) ribosomes; cycloheximide, a potent inhibitor of 80s translation completely abolished the synthesis, while the inhibitors of 70s translation chloramphenicol and erythromycin were not effective. In addition, ferredoxin was detected in a streptomycin-bleached mutant that lacks the chloroplast structure and chloroplast DNA. 2. Ferredoxin's transcript is presumably of nuclear origin; rifampicin, an inhibitor of chloroplast DNA-dependent RNA polymerase did not inhibit synthesis, while the streptomycin-bleached mutant continued to synthesize ferredoxin without chloroplast DNA.

Inhibition of chloroplast development in Euglena by streptomycin: Differential inhibition of the appearance of photosynthesis in the presence of the continued synthesis of chlorophyll

Streptomycin effectively inhibits chloroplast development in dark-grown non-dividing Euglena gracilis when added at the onset of greening but apparently exerts a diminished effect on chlorophyll synthesis when added at later stages. We have further investigated this phenomenon and show that streptomycin added to a Euglena culture at 24 h of development has a differential inhibitory action on the extent of synthesis of several chloroplast-associated parameters. Chlorophyll, carotenoids, cytochrome 552 and photosystem I Hill activity are all slightly, if at all, inhibited and to approximately the same extent between 24 and 72 h of development. We find a very strong inhibition of both ribulose diphosphate carboxylase synthesis and photosystem II Hill activity.

A reinvestigation of the sites of transcription and translation of Euglena chloroplastic phenylalanyl-tRNA synthetase

An attempt was made to determine the sites of chloroplast phenylalanyl-tRNA synthetase transcription and translation. Inhibitors of bacterial RNA and protein synthesis were added to logarithmic and stationary phase cultures of Euglena gracilis wild-type B. Logarithmic phase cultures were sensitive to both types of inhibitors. In stationary phase cultures plastid synthetase was reduced by RNA but not by protein synthesis inhibitors. The effect of the antibiotics on the mitochondrial enzyme was also noted. Several possible explanations of these resuults are discussed.

Amino acid incorporation into protein by ribosomes bound to chloroplast thylakoid membranes: formation of discrete products

A system which incorporates amino acids into proteins of chloroplast membranes of Chlamydomonas reinhardti is described. It consists of chloroplast ribosomes bound to thylakoid membranes and cell extract. mRNA is present in this thylakoid-ribosome complex, since neither initiation nor RNA synthesis seems to be required for amino acid incorporation. Incorporation requires ATP, GTP and a soluble portion of cell extract. It is inhibited by chloramphenicol, but not cycloheximide. Most incorporated radioactivity remains bound to the membranes. Although a large portion of this labeled membrane-bound protein occurs as nascent polypeptides, a portion appears at least four products of discrete molecular weights. The major in vitro product migrates as a polypeptide of 23 000 daltons. We conclude that a portion of chloroplast membrane proteins is not only made within the chloroplast, but directly on the membranes. We had previously observed that release of membrane-bound ribosomes is partially dependent on puromycin, and concluded that some membrane-bound ribosomes were attached to the membranes through nascent protein chains. Thus, our results suggest that some chloroplast membrane proteins are inserted into the membranes as they are synthesized. This chloroplast membrane amino acid incorporation system offers a promising tool for studying biosynthesis of membrane proteins, and how they become inserted into chloroplast thylakoids to form functional membranes.