The chloroplast-located glutamine synthetase of Phaseolus vulgaris L.: nucleotide sequence,expression in different organs and uptake into isolated chloroplasts

Work using a full-length cDNA clone has revealed that the plastid-located glutamine synthetase (GS) of Phaseolus vulgaris is encoded by a single nuclear gene. Nucleotide sequencing has shown that this cDNA is more closely related to a cDNA encoding the plastidic GS of Pisum sativum than to cDNAs encoding three different cytosolic GS subunits of P. vulgaris. The plastid GS subunits are initially synthesized as higher M _r (47000) precursors containing an N-terminal presequence of about 50 amino acids which is structurally similar to the presequences of other nuclear-encoded chloroplast proteins. The precursor has been synthesized in vitro and is imported by isolated pea chloroplasts and processed to two polypeptides of the same size as native P. vulgaris chloroplast GS subunits (M _r 42000). Experiments with fusion proteins show that the N-terminal 68 amino acids of this precursor allow the cytosolic GS subunit also to be imported and processed by isolated chloroplasts. Polyadenylated mRNA specifically related to the plastidic GS gene is most highly abundant in chloroplast-containing organs (leaves and stems) but is also detectable in roots and nodules.