Presence in the stroma of chloroplasts of a large pool of a ribosomal protein not structurally related to any Escherichia coli ribosomal protein

Molecular genetics of chloroplast ribosomal proteins

Chloroplasts contain a complete translational apparatus which, in land plants, synthesizes the 80 or so polypeptides encoded by the organelle's own small genome. Recent molecular genetic studies have revealed much about the chloroplast ribosomal proteins (RPs). Some of these proteins are encoded by the chloroplast genome and others by the nuclear genome. Many of these genes have now been cloned and characterized, including some that have no prokaryotic homologues.

Expression and functional assembly into bacterial ribosomes of a nuclear-encoded chloroplast ribosomal protein with a long NH2-terminal extension

Chloroplast ribosomal protein L13 is encoded in the plant nucleus and is considerably larger than its eubacterial homologue by having NH2- and COOH-terminal extensions with no homology to any known sequences (Phua et al., J Biol. Chem. 264, 1968-1971, 1989). We made two gene constructs of L13 cDNA using the polymerase chain reaction (PCR) and expressed them in Escherichia coli. Analysis of the ribosomes and polysomes from these cells, using an antiserum specific to chloroplast L13, shows that the expressed proteins are incorporated, in the presence of the homologous E. coli L13, into functional ribosomes which participate in protein synthesis (i.e. polysomes). Evidence is obtained that the large NH2-terminal extension probably lies on the surface of these 'mosaic ribosomes.' This first report of the assembly into E. coli ribosomes of nuclear-coded chloroplast ribosomal protein with terminal extensions thus suggest an extraordinary conservation in the function of eubacterial type ribosomal proteins, despite the many changes in protein structure during their evolution inside a eukaryotic system.

The nuclear:organelle distribution of chloroplast ribosomal proteins genes. Features of a cDNA clone encoding the cytoplasmic precursor of L11

The majority of chloroplast ribosomal proteins are encoded in the nuclear genome. In order to characterize these proteins through their mRNA, we have previously constructed a spinach cDNA expression library and raised antisera to several spinach chloroplast ribosomal proteins. Here we describe the immuno isolation of cDNA clones encoding protein L11 and its chloroplast-targeting presequence. The cytoplasmic precursor form of L11 is 224 amino acid residues long (Mr 23,662); the mature L11 and the transit sequence are predicted to be of approximately 159 and approximately 65 residues, respectively. The predicted chloroplast L11 is significantly longer than the E coli L11, but similar (in size) to archaebacterial and yeast cytoplasmic L11. In sequence it is closer to E coli L11 (54% identity) than to the archaebacterial (32%) or yeast (23%) proteins. These results and the conservation of the contexts of the 3 methyl modified residues found in E coli L11 are discussed in the light of the endosymbiont theory and nuclear relocation of the rp/KAJL gene cluster.

Chloroplast ribosomal protein L15, like L1, L13 and L21, is significantly larger than its E. coli homologue

The purification and identification by peptide sequence and immunological data of the spinach chloroplast homologue of E. coli L15 is presented. A significant increase in its mass over the E. coli counterpart is shown and is accounted for, in part, by a sequenced 18-residue N-terminal extension. A still larger C-terminal extension or internal insertion(s) is inferred. The migration position of the L15 in a 2D gel pattern of spinach chloroplast 50S subunit proteins is shown. Lack of sequence identity with the known chloroplast genomic data confirms the nuclear coding of this protein, and the N-terminal sequence given here provides the transit peptide cleavage site of the cytoplasmic precursor.