Guanosine tetra- and pentaphosphate synthase activity in chloroplasts of a higher plant: association with 70S ribosomes and inhibition by tetracycline

Chloroplasts possess bacterial-type systems for transcription and translation. On the basis of the identification of a Chlamydomonas reinhardtii gene encoding a RelA-SpoT homolog (RSH) that catalyzes the synthesis of guanosine tetra- or pentaphosphate [(p)ppGpp], we have previously suggested the operation of stringent control in the chloroplast genetic system. Although RSH genes have also been identified in several higher plants, the activities of the encoded enzymes and their mode of action in chloroplasts have remained uncharacterized. We have now characterized the intrinsic (p)ppGpp synthase activity of chloroplast extracts prepared from pea (Pisum sativum). Fractionation by ultracentrifugation suggested that the (p)ppGpp synthase activity of a translationally active chloroplast stromal extract was associated with 70S ribosomes. Furthermore, this enzymatic activity was inhibited by tetracycline, as was the peptide elongation activity of the extract. Structural comparisons between rRNA molecules of Escherichia coli and pea chloroplasts revealed the conservation of putative tetracycline-binding sites. These observations demonstrate the presence of a ribosome-associated (p)ppGpp synthase activity in the chloroplasts of a higher plant, further implicating (p)ppGpp in a genetic system of chloroplasts similar to that operative in bacteria.