title: Bacterial alarmone, guanosine 5'-diphosphate 3'-diphosphate (ppGpp), predominantly binds the beta' subunit of plastid-encoded plastid RNA polymerase in chloroplasts.
abstract: It's alarming: Bacterial alarmone guanosine 5'-diphosphate 3'-diphosphate (ppGpp), which is a key regulatory molecule that controls the stringent response, also exists in chloroplasts of plant cells. Cross-linking experiments with 6-thioguanosine 5'-diphosphate 3'-diphosphate (6-thioppGpp) and chloroplast RNA polymerase indicate that ppGpp binds the beta' subunit of plastid-encoded plastid RNA polymerase that corresponds to the Escherichia coli beta' subunit. Chloroplasts, which are thought to have originated from cyanobacteria, have their own genetic system that is similar to that of the bacteria from which they were derived. Recently, bacterial alarmone guanosine 5'-diphosphate 3'-diphosphate (ppGpp, 1), a key regulatory molecule that controls the stringent response, was identified in the chloroplasts of plant cells. Similar to its function in bacteria, ppGpp inhibits chloroplast RNA polymerase; this suggests that ppGpp mediates gene expression through the stringent response in chloroplasts. However, a detailed mechanism of ppGpp action in chloroplasts remains elusive. We synthesized 6-thioguanosine 5'-diphosphate 3'-diphosphate (6-thioppGpp) as a photoaffinity probe of ppGpp; this probe thus enabled the investigation of ppGpp binding to chloroplast RNA polymerase. We found that 6-thioppGpp, as well as ppGpp, inhibits chloroplast RNA synthesis in vitro in a dose-dependent manner. Cross-linking experiments with 6-thioppGpp and chloroplast RNA polymerase indicated that ppGpp binds the beta' subunit (corresponding to the Escherichia coli beta' subunit) of plastid-encoded plastid RNA polymerase composed of alpha, beta, beta', beta'', and sigma subunits. Furthermore, ppGpp did not inhibit transcription in plastid nucleoids prepared from tobacco BY-2 cells; this suggests that ppGpp does not inhibit nuclear-encoded plastid RNA polymerase.
authors.affiliation: Division of Applied Bioscience, Research Faculty of Agriculture, Hokkaido University, Kita-9, Nishi-9, Kita-ku, Sapporo 060-8589, Japan.
authors.forename: Michio
authors.initials: M
authors.lastname: Sato
authors.forename: Kosaku
authors.initials: K
authors.lastname: Takahashi
authors.forename: Yuka
authors.initials: Y
authors.lastname: Ochiai
authors.forename: Takeshi
authors.initials: T
authors.lastname: Hosaka
authors.forename: Kozo
authors.initials: K
authors.lastname: Ochi
authors.forename: Kensuke
authors.initials: K
authors.lastname: Nabeta
mesh_terms: D000595:Amino Acid Sequence; D002736:Chloroplasts / Q000201:enzymology* / Q000378:metabolism; D012321:DNA-Directed RNA Polymerases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D006159:Guanosine Tetraphosphate / Q000138:chemical synthesis / Q000737:chemistry / Q000494:pharmacology*; D018087:Plastids / Q000378:metabolism; D011485:Protein Binding; D017386:Sequence Homology, Amino Acid
chemical_list: D006159:Guanosine Tetraphosphate; D012321:DNA-Directed RNA Polymerases