Nucleotide sequence and linkage map position of the genes for ribosomal proteins L14 and S8 in the maize chloroplast genome

Mutagenesis of ribosomal protein S8 from Escherichia coli: expression, stability, and RNA-binding properties of S8 mutants

Protein S8, a 129 amino acid component of the Escherichia coli ribosome, plays an essential role in the assembly of the 30S ribosomal subunit and in the translational regulation of the spc operon by virtue of its capacity to bind specifically to rRNA and mRNA. To study structure-function relationships within the protein, we have constructed a vector for its high-level expression in vivo and developed efficient methods for its purification. Under our conditions, S8 accumulates to a level of 35% of the cellular protein and can be prepared at a purity of over 98% using either HPLC or a combination of ion-exchange and gel-filtration chromatography. The unique cysteine residue at position 126 was replaced by alanine or serine by oligonucleotide-directed mutagenesis, and the two mutant proteins, CA126 and CS126, were expressed and isolated. The effects of the mutations on the RNA-binding ability, secondary structure, and stability of S8 were assessed. CD spectra indicated that wild-type S8 and the two mutant proteins have very similar secondary structures at 25 degrees C. In addition, both mutants are metabolically stable in vivo as inferred from pulse-chase labeling and immunoprecipitation experiments. However, while CA126 exhibits the same affinity for RNA and the same susceptibility to urea and thermal denaturation as wild-type S8, CS126 is severely impaired in its ability to interact with RNA and displays a dramatic reduction in conformational stability. Our results suggest that Cys126 is unlikely to play a specific role in RNA recognition but that it is an integral part of the RNA-binding domain of protein S8.

Nucleotide sequence of maize chloroplast rpl32: completing the apparent set of plastid ribosomal protein genes and their tentative operon organization

By sequencing the rpl32 gene, we have characterized the apparent complete set of the RP genes in Zea mays plastid genome. Key data for these 21 genes (total of 26 gene copies) and the proteins encoded by them are presented, and the operon organization is discussed on the basis of available transcription data. A nomenclature for the inferred 13 operons is suggested.

Mutagenesis of ribosomal protein S8 from Escherichia coli: defects in regulation of the spc operon

The structural features of Escherichia coli ribosomal protein S8 that are involved in translational regulation of spc operon expression and, therefore, in its interaction with RNA have been investigated by use of a genetic approach. The rpsH gene, which encodes protein S8, was first inserted into an expression vector under the control of the lac promoter and subsequently mutagenized with methoxylamine or nitrous acid. A screening procedure based on the regulatory role of S8 was used to identify mutants that were potentially defective in their ability to associate with spc operon mRNA and, by inference, 16S mRNA. In this way, we isolated 39 variants of the S8 gene containing alterations at 34 different sites, including 37 that led to single amino acid substitutions and 2 that generated premature termination codons. As the mutations were distributed throughout the polypeptide chain, our results indicate that amino acid residues important for the structural integrity of the RNA-binding domain are not localized to a single segment. Nonetheless, the majority were located within three short sequences at the N terminus, middle, and C terminus that are phylogenetically conserved among all known eubacterial and chloroplast versions of this protein. We conclude that these sites encompass the main structural determinants required for the interaction of protein S8 with RNA.

The primary structure of rat ribosomal protein L23

The amino acid sequence of the rat 60S ribosomal subunit protein L23 was deduced from the sequence of nucleotides in two recombinant cDNAs. Ribosomal protein L23 has 140 amino acids and a molecular weight of 14,856. Hybridization of the cDNA to digests of nuclear DNA suggests that there are 7-9 copies of the L23 gene. The mRNA for the protein is about 600 nucleotides in length. Rat L23 is homologous to Saccharomyces cerevisiae L17a and related to Escherichia coli L14 and other members of the prokaryotic L14 family.

Isolation and analysis of a human cDNA highly homologous to the yeast gene encoding L17A ribosomal protein

A cDNA from human brain poly(A)+RNA with significant similarity to the gene encoding yeast L17A large subunit ribosomal (r) protein (L17A) was isolated using the polymerase chain reaction. The deduced amino acid (aa) sequence of 140 aa (calculated pI of 10.79) exhibits a 78% similarity to that of the yeast L17A r protein (88% when conservative aa replacements are considered as well). This indicates that L17A is one of the best conserved r-proteins and therefore may play a critical role in ribosome function. In contrast to its eubacterial and chloroplast counterparts, human L17A contains an N-terminal extension of 19 aa which may be involved in nuclear targeting of the r-protein. Approximately five to seven genes in mammalian genomes give strong hybridization signals when probed with the human L17A homologue cDNA. Whereas the L17A homologue was found to be expressed at similar levels in several human tissues as a transcript of 600 nucleotides, a several-fold higher transcript level was detected in the rapidly growing neuroblastoma cell line, SK-N-BE.

Extended N-terminal sequencing of proteins of the large ribosomal subunit from yeast mitochondria

We have determined the N-termini of 26 proteins of the large ribosomal subunit from yeast mitochondria by direct amino acid micro-sequencing. The N-terminal sequences of proteins YmL33 and YmL38 showed a significant similarity to eubacterial ribosomal (r-) proteins L30 and L14, respectively. In addition, several proteins could be assigned to their corresponding yeast nuclear genes. Based on a comparison of the protein sequences deduced from the corresponding DNA regions with the N-termini of the mature proteins, the putative leader peptides responsible for mitochondrial matrix-targeting were compiled. In most leader sequences a relative abundance of aromatic amino acids, preferentially phenylalanine, was found.

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.

Cloning and characterization of chloroplast ribosomal protein-encoding genes, rpl16 and rps3, of the marine macro-algae, Gracilaria tenuistipitata

In Gracilaria tenuistipitata, a highly differentiated multicellular member of the marine red algae, Rhodophyta, chloroplast (cp) DNA can be separated as a satellite band from the nuclear DNA in a CsCl gradient. Using a heterologous probe from Chlamydomonas, the ribosomal protein-encoding gene, rpl16, was located on a 4.5-kb EcoRI fragment of cp DNA. The fragment was cloned and a 1365-bp region around rpl16 was sequenced. The gene order around rpl16, 5' rpl22-rps3-rpl16, is identical to that detected in the chloroplast DNA of liverwort, tobacco and maize. Both the nucleotide sequence and the amino-acid sequence of rpl16 are more conserved than that of rps3. The rpl16 gene contains no intron, a feature which shows more similarity to the unicellular green algae, Chlamydomonas, than to other land plants. Sequences that may form a stable stem-loop structure were detected within the coding sequence of rpl16.

Chloroplast genes transferred to the nuclear plant genome have adjusted to nuclear base composition and codon usage

During plant evolution, some plastid genes have been moved to the nuclear genome. These transferred genes are now correctly expressed in the nucleus, their products being transported into the chloroplast. We compared the base compositions, the distributions of some dinucleotides and codon usages of transferred, nuclear and chloroplast genes in two dicots and two monocots plant species. Our results indicate that transferred genes have adjusted to nuclear base composition and codon usage, being now more similar to the nuclear genes than to the chloroplast ones in every species analyzed.

Euglena gracilis chloroplast ribosomal protein operon: a new chloroplast gene for ribosomal protein L5 and description of a novel organelle intron category designated group III

We describe the structure (3840 bp) of a novel Euglena gracilis chloroplast ribosomal protein operon that encodes the five genes rpl16-rpl14-rpl5-rps8-rpl36. The gene organization resembles the spc and the 3'-end of the S10 ribosomal protein operons of E. coli. The rpl5 is a new chloroplast gene not previously reported for any chloroplast genome to date and also not described as a nuclear-encoded, chloroplast protein gene. The operon contains at least 7 introns. We present evidence from primer extension analysis of chloroplast RNA for the correct in vivo splicing of five of the introns. Two of the introns within the rps8 gene flank an 8 bp exon, the smallest exon yet characterized in a chloroplast gene. Three introns resemble the classical group II introns of organelle genomes. The remaining 4 introns appear to be unique to the Euglena chloroplast DNA. They are uniform in size (95-109 nt), share common features with each other and are distinct from both group I and group II introns. We designate this new intron category as 'group III'.

Translation of chloroplast-encoded mRNA: potential initiation and termination signals

A survey of 196 protein-coding chloroplast DNA sequences demonstrated the preference for AUG and UAA codons for initiation and termination of translation, respectively. As in prokaryotes at every nucleotide position from -25 to +25 (AUG is +1 to +3) and for 25 nucleotides 5' and 3' to the termination codon an A or U is predominant, except for C at +5 and G at +22. A Shine-Dalgarno (SD) sequence (GGAGG or tri- or tetranucleotide variant) was found within 100 bp 5' to the AUG codon in 92% of the genes. In 40% of these cases, the location of the SD sequence was similar to that of the consensus for prokaryotes (-12 to -7 5' to AUG), presumed to be optimal for translation initiation. A SD sequence could not be located in 6% of the chloroplast sequences. We propose that mRNA secondary structures may be required for the relocation of a distal SD sequences to within the optimal region (-12 to -7) for initiation of translation. We further suggest that termination at UGA codons in chloroplast genes may occur by a mechanism, involving 16S rRNA secondary structure, which has been proposed for UGA termination in E. coli.