Lincomycin as a chloroplast probe

ppGpp inhibits peptide elongation cycle of chloroplast translation system in vitro

Chloroplasts possess common biosynthetic pathways for generating guanosine 3',5'-(bis)pyrophosphate (ppGpp) from GDP and ATP by RelA-SpoT homolog enzymes. To date, several hypothetical targets of ppGpp in chloroplasts have been suggested, but they remain largely unverified. In this study, we have investigated effects of ppGpp on translation apparatus in chloroplasts by developing in vitro protein synthesis system based on an extract of chloroplasts isolated from pea (Pisum sativum). The chloroplast extracts showed stable protein synthesis activity in vitro, and the activity was sensitive to various types of antibiotics. We have demonstrated that ppGpp inhibits the activity of chloroplast translation in dose-effective manner, as does the toxic nonhydrolyzable GTP analog guanosine 5'-(β,γ-imido)triphosphate (GDPNP). We further examined polyuridylic acid-directed polyphenylalanine synthesis as a measure of peptide elongation activity in the pea chloroplast extract. Both ppGpp and GDPNP as well as antibiotics, fusidic acid and thiostrepton, inhibited the peptide elongation cycle of the translation system, but GDP in the similar range of the tested ppGpp concentration did not affect the activity. Our results thus show that ppGpp directly affect the translation system of chloroplasts, as they do that of bacteria. We suggest that the role of the ppGpp signaling system in translation in bacteria is conserved in the translation system of chloroplasts.

[Structure and function of the genetic information in plastids : V. The absence of ribosomal rna from the plastids of the plastom-mutant 'mrs. parker' of Pelargonium zonale]

1. The variety 'Mrs. Parker' of Pelargonium zonale (fig. 1) is a periclinal chimera of the constitution white-over-green (LI and L II: white, L III: green). Reciprocal crosses with the green variety 'Trautlieb' demonstrate a biparental, extranuclear inheritance of the character green.- white. The F1 consists of green, green-white variegated and white seedlings (table 1). 2. In green-white variegated F1-plants "mixed cells" (fig. 2) have been found containing two genetically different types of plastids: green plastids (from 'Trautlieb') and white plastids (from 'Mrs. Parker'). The white cells of 'Mrs. Parker' represent a white plastid mutant (= plastom mutant); its genetic designation is "extranuclear: alba-1", symbol en: alba-1. 3. Leaf material for biochemical studies was obtained from pure white and entirely green shoots of variegated F1 hybrids (fig. 3). The ultrastructure of the white plastids was studied by electron microscopy. 4. From normal green cells of Pelargonium zonale four bands of high molecular weight ribosomal RNA can be isolated: 25 S and 18 S RNA of the cytoplasmic ribosomes and 23 S and 16 S RNA of plastid ribosomes (fig. 5a). 5. The mutation of the plastid DNA in the plastids of 'Mrs. Parker' causes an altered RNA pattern: The 23 S and 16 S RNA of the plastid ribosomes cannot be detected in polyacrylamid gels (whereas 25 S and 18 S RNA are present) (fig. 5b). 6. In cells of white leaves numerous plastids are present. They are smaller than normal chloroplasts and have a double-layered envelope. However, the formation of normal internal membrane structures is blocked. 7. In mutated plastids of 'Mrs. Parker' ribosomes cannot be detected in electron micrographs (fig. 6). 8. From these findings we conclude that protein synthesis cannot be performed in mutated plastids. The multiplication of the plastids - and presumably also the replication of plastid DNA - is not impaired by the deficiency in plastid protein synthesis. 9. These results indicate that protein synthesis within the plastids is necessary for full development and differentiation of the chloroplasts, although an essential part of the plastidal proteins are synthesized on cytoplasmic ribosomes.