Transcriptional organization of the S10, spc and alpha operons of Escherichia coli

The Dynamic SecYEG Translocon

The spatial and temporal coordination of protein transport is an essential cornerstone of the bacterial adaptation to different environmental conditions. By adjusting the protein composition of extra-cytosolic compartments, like the inner and outer membranes or the periplasmic space, protein transport mechanisms help shaping protein homeostasis in response to various metabolic cues. The universally conserved SecYEG translocon acts at the center of bacterial protein transport and mediates the translocation of newly synthesized proteins into and across the cytoplasmic membrane. The ability of the SecYEG translocon to transport an enormous variety of different substrates is in part determined by its ability to interact with multiple targeting factors, chaperones and accessory proteins. These interactions are crucial for the assisted passage of newly synthesized proteins from the cytosol into the different bacterial compartments. In this review, we summarize the current knowledge about SecYEG-mediated protein transport, primarily in the model organism Escherichia coli, and describe the dynamic interaction of the SecYEG translocon with its multiple partner proteins. We furthermore highlight how protein transport is regulated and explore recent developments in using the SecYEG translocon as an antimicrobial target.

A Small Protein Unique to Bacteria Organizes rRNA Tertiary Structure Over an Extensive Region of the 50S Ribosomal Subunit

A number of small, basic proteins penetrate into the structure of the large subunit of the ribosome. While these proteins presumably aid in the folding of the rRNA, the extent of their contribution to the stability or function of the ribosome is unknown. One of these small, basic proteins is L36, which is highly conserved in Bacteria, but is not present in Archaea or Eucarya. Comparison of ribosome crystal structures shows that the space occupied by L36 in a bacterial ribosome is empty in an archaeal ribosome. To ask what L36 contributes to ribosome stability and function, we have constructed an Escherichia coli strain lacking ribosomal protein L36; cell growth is slowed by 40-50% between 30 degrees C and 42 degrees C. Ribosomes from this deletion strain sediment normally and have a full complement of proteins, other than L36. Chemical protection experiments comparing rRNA from wild-type and L36-deficient ribosomes show the expected increase in reagent accessibility in the immediate vicinity of the L36 binding site, but suggest that a cooperative network of rRNA tertiary interactions has been disrupted along a path extending 60 A deep into the ribosome. These data argue that L36 plays a significant role in organizing 23 S rRNA structure. Perhaps the Archaea and Eucarya have compensated for their lack of L36 by maintaining more stable rRNA tertiary contacts or by adopting alternative protein-RNA interactions elsewhere in the ribosome.

Regulation of ribosomal protein synthesis in Vibrio cholerae

We have investigated the regulation of the S10 and spc ribosomal protein (r-protein) operons in Vibrio cholerae. Both operons are under autogenous control; they are mediated by r-proteins L4 and S8, respectively. Our results suggest that Escherichia coli-like strategies for regulating r-protein synthesis extend beyond the enteric members of the gamma subdivision of proteobacteria.

Ribosomal RNA and ribosomal proteins in corynebacteria

Ribosomal RNAs (rRNAs) (16S, 23S, 5S) encoded by the rrn operons and ribosomal proteins play a very important role in the formation of ribosomes and in the control of translation. Five copies of the rrn operon were reported by hybridization studies in Brevibacterium (Corynebacterium) lactofermentum but the genome sequence of Corynebacterium glutamicum provided evidence for six rrn copies. All six copies of the C. glutamicum 16S rRNA have a size of 1523 bp and each of the six copies of the 5S contain 120 bp whereas size differences are found between the six copies of the 23S rRNA. The anti-Shine-Dalgarno sequence at the 3'-end of the 16S rRNA was 5'-CCUCCUUUC-3'. Each rrn operon is transcribed as a large precursor rRNA (pre-rRNA) that is processed by RNaseIII and other RNases at specific cleavage boxes that have been identified in the C. glutamicum pre-rRNA. A secondary structure of the C. glutamicum 16S rRNA is proposed. The 16S rRNA sequence has been used as a molecular evolution clock allowing the deduction of a phylogenetic tree of all Corynebacterium species. In C. glutamicum, there are 11 ribosomal protein gene clusters encoding 42 ribosomal proteins. The organization of some of the ribosomal protein gene cluster is identical to that of Escherichia coli whereas in other clusters the organization of the genes is rather different. Some specific ribosomal protein genes are located in a different cluster in C. glutamicum when compared with E. coli, indicating that the control of expression of these genes is different in E. coli and C. glutamicum.

Gene content and organization of an 85-kb DNA segment from the genome of the phytopathogenic mollicute Spiroplasma kunkelii

Spiroplasma kunkelii, the causative agent of corn stunt disease in maize (Zea maysL.), is a helical, cell wall-less prokaryote assigned to the class Mollicutes. As part of a project to sequence the entire S. kunkelii genome, we analyzed an 85-kb DNA segment from the pathogenic strain CR2-3x. This genome segment contains 101 ORFs and two tRNA genes. The majority of the ORFs code for predicted proteins that can be assigned to respective clusters of orthologous groups (COGs). These COGs cover diverse functional categories including genetic information storage and processing, cellular processes, and metabolism. The most notable gene cluster in this genome segment is a super-operon capable of encoding 24 ribosomal proteins. The organization of genes in this operon reflects the unique evolutionary position of the spiroplasma. Gene duplications, domain rearrangements, and frameshift mutations in the segment are interpreted as indicators of phase variation in the spiroplasma. To our knowledge, this is the first analysis of a large genome segment from a plant pathogenic spiroplasma.

Ribosomal protein S4 is a transcription factor with properties remarkably similar to NusA, a protein involved in both non-ribosomal and ribosomal RNA antitermination

Escherichia coli ribosomal RNA (rRNA) operons contain antitermination motifs necessary for forming terminator-resistant transcription complexes. In preliminary work, we isolated 'antiterminating' transcription complexes and identified four new proteins potentially involved in rRNA transcription antitermination: ribosomal (r-) proteins S4, L3, L4 and L13. We show here that these r-proteins and Nus factors lead to an 11-fold increase in terminator read-through in in vitro transcription reactions. A significant portion of the effect was a result of r-protein S4. We show that S4 acted as a general antitermination factor, with properties very similar to NusA. It retarded termination and increased read-through at Rho-dependent terminators, even in the absence of the rRNA antiterminator motif. High concentrations of NusG showed reduced antitermination by S4. Like rrn antitermination, S4 selectively antiterminated at Rho-dependent terminators. Lastly, S4 tightly bound RNA polymerase in vivo. Our results suggest that, like NusA, S4 is a general transcription antitermination factor that associates with RNA polymerase during normal transcription and is also involved in rRNA operon antitermination. A model for key r-proteins playing a regulatory role in rRNA synthesis is presented.

Mutations in ribosomal protein L16 conferring reduced susceptibility to evernimicin (SCH27899): implications for mechanism of action

A clinical isolate of Streptococcus pneumoniae (SP#5) that showed decreased susceptibility to evernimicin (MIC, 1.5 microgram/ml) was investigated. A 4,255-bp EcoRI fragment cloned from SP#5 was identified by its ability to transform evernimicin-susceptible S. pneumoniae R6 (MIC, 0.03 microgram/ml) such that the evernimicin MIC was 1.5 microgram/ml. Nucleotide sequence analysis of this fragment revealed that it contained portions of the S10-spc ribosomal protein operons. The nucleotide sequences of resistant and susceptible isolates were compared, and a point mutation (thymine to guanine) that causes an Ile52-Ser substitution in ribosomal protein L16 was identified. The role of this mutation in decreasing susceptibility to evernimicin was confirmed by direct transformation of the altered L16 gene. The presence of the L16 mutation in the resistant strain suggests that evernimicin is an inhibitor of protein synthesis. This was confirmed by inhibition studies using radiolabeled substrates, which showed that the addition of evernimicin at sub-MIC levels resulted in a rapid decrease in the incorporation of radiolabeled isoleucine in a susceptible isolate (SP#3) but was much less effective against SP#5. The incorporation of isoleucine showed a linear response to the dose level of evernimicin. The incorporation of other classes of labeled substrates was unaffected or much delayed, indicating that these were secondary effects.

Characterization of the Leptospira interrogans S10-spc-alpha operon

A ribosomal protein gene cluster from the spirochaete Leptospira interrogans was characterized. This locus is homologous to the Escherichia coli S10, spc, and alpha operons. Analysis of L. interrogans RNA showed that the ribosomal protein genes within this cluster are co-transcribed, thus forming an operon. Two transcription initiation sites were mapped by primer extension, upstream of fus, the first gene in this cluster, and sequences from this region provided promoter activity in E. coli. Transcription terminates near a predicted stem-loop structure following rplQ, the last gene in the cluster. These data suggest that two promoters upstream of fus direct transcription of this 17.5-kb ribosomal protein gene cluster. Comparison of the L. interrogans S10-spc-alpha cluster to homologous loci from Borrelia burgdorferi and Treponema pallidum provided evidence that this region of the genome underwent several rearrangements during spirochaete evolution.

Novel intergenic repeats of Escherichia coli K-12

An online catalog of intergenic DNA repeat sequence elements is added to the EcoGene Escherichia coli K-12 genome sequence annotation and analysis project (bmb.med.miami.edu/EcoGene). A library of noncoding (intergenic) DNA sequences depleted of known intergenic repeat classes was searched for DNA sequence similarities to identify novel DNA repeat sequence classes.

Phylogenetic analysis of L4-mediated autogenous control of the S10 ribosomal protein operon

We investigated the regulation of the S10 ribosomal protein (r-protein) operon among members of the gamma subdivision of the proteobacteria, which includes Escherichia coli. In E. coli, this 11-gene operon is autogenously controlled by r-protein L4. This regulation requires specific determinants within the untranslated leader of the mRNA. Secondary structure analysis of the S10 leaders of five enterobacteria (Salmonella typhimurium, Citrobacter freundii, Yersinia enterocolitica, Serratia marcescens, and Morganella morganii) and two nonenteric members of the gamma subdivision (Haemophilus influenzae and Vibrio cholerae) shows that these foreign leaders share significant structural homology with the E. coli leader, particularly in the region which is critical for L4-mediated autogenous control in E. coli. Moreover, these heterologous leaders produce a regulatory response to L4 oversynthesis in E. coli. Our results suggest that an E. coli-like L4-mediated regulatory mechanism may operate in all of these species. However, the mechanism is not universally conserved among the gamma subdivision members, since at least one, Pseudomonas aeruginosa, does not contain the required S10 leader features, and its leader cannot provide the signals for regulation by L4 in E. coli. We speculate that L4-mediated autogenous control developed during the evolution of the gamma branch of proteobacteria.

Purification, cloning, and preliminary characterization of a Spiroplasma citri ribosomal protein with DNA binding capacity

The rpsB-tsf-x operon of Spiroplasma citri encodes ribosomal protein S2 and elongation factor Ts, two components of the translational apparatus, and an unidentified X protein. A potential DNA-binding site (a 20-base pair (bp) inverted repeat sequence) is located at the 3' end of rpsB. Southwestern analysis of S. citri proteins, with a 30-bp double-stranded oligonucleotide probe (IRS), containing the 20-bp inverted repeat sequence and the genomic flanking sequences, detected an IRS-binding protein of 46 kDa (P46). P46 protein, which displays preferential affinity for the IRS, was purified from S. citri by a combination of affinity and gel filtration chromatographies. The native form of P46 seems to be homomultimeric as estimated by SDS-polyacrylamide gel electrophoresis analysis and gel filtration. A 3.5-kilobase pair S. citri DNA fragment comprising the P46 gene and flanking sequences was cloned and sequenced. Sequence analysis of this DNA fragment indicated that the P46 gene is located within the S10-spc operon of S. citri at the position of the gene coding for ribosomal protein L29 in the known S10-spc operons. The similarity between the N-terminal domain of P46 and the L29 ribosomal protein family and the presence of a 46-kDa IRS-binding protein in S. citri ribosomes indicated that P46 is the L29 ribosomal protein of S. citri. We suggest that P46 is a bifunctional protein with an L29 N-terminal domain and a C-terminal domain involved in IRS binding.

Analysis of the Bacillus subtilis S10 ribosomal protein gene cluster identifies two promoters that may be responsible for transcription of the entire 15-kilobase S10-spc-alpha cluster

We have sequenced a previously uncharacterized region of the Bacillus subtilis S10 ribosomal protein gene cluster. The new segment includes genes for S10, L3, L4, L23, L2, S19, L22, S3, and part of L16. These B. subtilis genes map in the same order as the genes in the Escherichia coli S10 ribosomal protein operon. Two potential promoter sequences were identified, one approximately 200 bases and the other approximately 140 bases upstream of the S10 gene. The activities of the two promoters were demonstrated by primer extension analysis, in vitro transcription experiments, and in vivo promoter fusion plasmid studies. In agreement with previous reports, our Northern analysis of exponentially growing cells failed to identify terminators or other active promoters within the S10-spc-alpha region. Our observations suggest that the two S10 promoters reported here are responsible for transcribing a 15-kb-long transcript for all of the genes in the B. subtilis S10, spc, and alpha clusters.

Sequencing and analysis of the Thermus thermophilus ribosomal protein gene cluster equivalent to the spectinomycin operon

To assess the organization of the Thermus thermophilus ribosomal protein genes, a fragment of DNA containing the complete S10 region and ten ribosomal protein genes of the spc region was cloned, using an oligonucleotide coding for the N-terminal amino acid (aa) sequence of T. thermophilus S8 protein as hybridization probe. The nucleotide sequence of a 4290 bp region between the rps17 and rpl15 genes was determined. Comparative analysis of this gene cluster showed that the gene arrangement (S17, L14, L24, L5, S14, S8, L6, L18, S5, L30 and L15) is identical to that of eubacteria. However, T. thermophilus ribosomal protein genes corresponding to the Escherichia coli S10 and spc operons are not resolved into two clusters: the stop codon of the rps17 gene (the last gene of the S10 operon in E. coli) and the start codon of the rpl14 gene (the first gene of the spc operon in E. coli) overlap. Most genes, except the rps14-rps8 intergenic spacer (69 bp), are separated by very short (only 3-7 bp) spacer regions or partially overlapped. The deduced aa sequences of T. thermophilus proteins share about 51-100% identities with the sequences of homologous proteins from thermophile Thermus aquaticus and Thermotoga maritima and 27-70% identities with the sequences of their mesophile counterparts.

Genome plasticity as a paradigm of eubacteria evolution

To test the hypotheses that eubacterial genomes leave evolutionarily stable structures and that the variety of genome size is brought about through genome doubling during evolution, the genome structures of Haemophilus influenzae, Mycoplasma genitalium, Escherichia coli, and Bacillus subtilis were compared using the DNA sequences of the entire genome or substantial portions of genome. In these comparisons, the locations of orthologous genes were examined among different genomes. Using orthologous genes for the comparisons guaranteed that differences revealed in physical location would reflect changes in genome structure after speciation. We found that dynamic rearrangements have so frequently occurred in eubacterial genomes as to break operon structures during evolution, even after the relatively recent divergence between E. coli and H. influenzae. Interestingly, in such eubacterial genomes of high plasticity, we could find several highly conservative regions with the longest conserved region comprising the S10, spc, and alpha operons. This suggests that such exceptional conservative regions have undergone strong structural constraints during evolution.

Genetic and transcriptional organization of the Bacillus subtilis spc-alpha region

We used chromosomal walking methods to isolate a 10.8-kb region from the major ribosomal protein (r-protein) gene cluster of Bacillus subtilis (Bs). The gene order in this region, given by gene product, was r-proteins L16-L29-S17-L14-L24-L5-S14-S8-L6-L18-S5-L30-L15-SecY-adenylate kinase (Adk)-methionine aminopeptidase (Map)-initiation factor 1 (IF1)-L36-S13-S11-alpha subunit of RNA polymerase-L17. The region cloned, therefore, contains the homologues for the last three genes of the Escherichia coli (Ec) S10 operon, together with entire spc and alpha operons. This Bs organization differs from the corresponding region in Ec by the inclusion of the genes encoding Adk, Map and IF1 between the genes encoding SecY and L36. Plasmid integration experiments indicated that all 22 genes comprise a single large transcriptional unit controlled from a major promoter which lies upstream from the gene encoding r-protein L16. Promoter probe experiments located lesser activities internal to this large transcriptional unit, the secY and map promoters. The secY promoter region (psecY) contained two activities, each principally functioning in the stationary growth phase when high protein export is required. Thus, the Bs S10-spc-alpha region differs from its Ec counterpart in both genetic and transcriptional organization. Given this difference in transcriptional organization, the mechanisms coordinating expression of the translational apparatus are also likely to differ between Ec and Bs.

Regulation of human RPS14 transcription by intronic antisense RNAs and ribosomal protein S14

RNase protection studies reveal two stable RNAs (250 and 280 nucleotides) transcribed from the antisense strand of the human ribosomal protein gene RPS14's first intron. These transcripts, designated alpha-250 and alpha-280, map to overlapping segments of the intron's 5' sequence. Neither RNA encodes a polypeptide sequence, and both are expressed in all human cells and tissues examined. Although alpha-280 is detected among both the cells' nuclear and cytoplasmic RNAs, the great majority of alpha-250 is found in the cytoplasmic subcellular compartment. As judged by its resistance to high concentrations of alpha-amanitin, cell-free transcription of alpha-250 and alpha-280 appears to involve RNA polymerase I. Tissue culture transfection and cell-free transcription experiments demonstrate that alpha-250 and alpha-280 stimulate S14 mRNA transcription, whereas free ribosomal protein S14 inhibits it. Electrophoretic mobility shift experiments indicate specific binary molecular interactions between r-protein S14, its message and the antisense RNAs. In light of these data, we propose a model for fine regulation of human RPS14 transcription that involves RPS14 intron 1 antisense RNAs as positive effectors and S14 protein as a negative effector.

Phylogenetic depth of S10 and spc operons: cloning and sequencing of a ribosomal protein gene cluster from the extremely thermophilic bacterium Thermotoga maritima

A segment of Thermotoga maritima DNA spanning 6,613 bp downstream from the gene tuf for elongation factor Tu was sequenced by use of a chromosome walking strategy. The sequenced region comprised a string of 14 tightly linked open reading frames (ORFs) starting 50 bp downstream from tuf. The first 11 ORFs were identified as homologs of ribosomal protein genes rps10, rpl3, rpl4, rpl23, rpl2, rps19, rpl22, rps3, rpl16, rpl29, and rps17 (which in Escherichia coli constitute the S10 operon, in that order); the last three ORFs were homologous to genes rpl14, rpl24, and rpl5 (which in E. coli constitute the three promoter-proximal genes of the spectinomycin operon). The 14-gene string was preceded by putative -35 and -10 promoter sequences situated 5' to gene rps10, within the 50-bp spacing between genes tuf and rps10; the same region exhibited a potential transcription termination signal for the upstream gene cluster (having tuf as the last gene) but displayed also the potential for formation of a hairpin loop hindering the terminator; this suggests that transcription of rps10 and downstream genes may start farther upstream. The similar organization of the sequenced rp genes in the deepest-branching bacterial phyla (T. maritima) and among Archaea has been interpreted as indicating that the S10-spc gene arrangement existed in the (last) common ancestor. The phylogenetic depth of the Thermotoga lineage was probed by use of r proteins as marker molecules: in all except one case (S3), Proteobacteria or the gram-positive bacteria, and not the genus Thermotoga, were the deepest-branching lineage; in only two cases, however, was the inferred branching order substantiated by bootstrap analysis.

Cloning and characterization of the RNA polymerase alpha-subunit operon of Chlamydia trachomatis

We have cloned the chlamydial operon that encodes the initiation factor IF1, the ribosomal proteins L36, S13, and S11, and the alpha subunit of RNA polymerase. The genes for S11 and alpha are closely linked in Escherichia coli, Bacillus subtilis, and plant chloroplast genomes, and this arrangement is conserved in Chlamydia spp. The S11 ribosomal protein gene potentially encodes a protein of 125 amino acids with 41 to 42% identity over its entire length to its E. coli and B. subtilis homologs; the gene encoding the alpha subunit specifies a protein of 322 amino acids with 25 to 30% identity over its entire length to its E. coli and B. subtilis homologs. In a T7-based expression system in E. coli, the chlamydial alpha gene directed the synthesis of a 36-kDa protein. Mapping of the chlamydial mRNA transcript by RNase protection studies and by a combination of reverse transcription and the polymerase chain reaction demonstrates that IF1, L36, S13, S11, and alpha are transcribed as a polycistronic transcript.

Cloning and sequence analysis of the Chlamydia trachomatis spc ribosomal protein gene cluster

We identified and sequenced a segment of Chlamydia trachomatis chromosomal DNA that shows homology to the Escherichia coli spc and distal region of the S10 ribosomal protein (r-protein) operons. Its sequence revealed a high degree of nucleotide and operon context conservation with the E. coli r-protein genes. The C. trachomatis spec operon contains the r-protein genes for L14, L24, L5, S8, L6, L18, S5, L15, and Sec Y along with the genes for r-proteins L16, L29, and S17 of the S10 operon. The two operons are separated by a 16-bp intragenic region which contains no transcription signals. However, a putative promoter for the transcription of the spc operon was found 162 nucleotides upstream of the CtrL14e start site; it revealed significant homology to the E. coli consensus promoter sequences. Interestingly, our results indicate the absence of any structure resembling an EcoS8 regulatory target site on C. trachomatis spc mRNA in spite of significant amino acid identity between E. coli and C. trachomatis r-proteins. Also, the intrinsic aminoglycoside resistance in C. trachomatis is unlikely to be mediated by CtrL6e since E. coli expressing CtrL6e remained susceptible to gentamicin (MIC less than 0.5 micrograms/ml).