Construction and Initial Characterization of Escherichia coli Strains with Few or No Intact Chromosomal rRNA Operons

The Escherichia coli genome carries seven rRNA (rrn) operons, each containing three rRNA genes. The presence of multiple operons has been an obstacle to many studies of rRNA because the effect of mutations in one operon is diluted by the six remaining wild-type copies. To create a tool useful for manipulating rRNA, we sequentially inactivated from one to all seven of these operons with deletions spanning the 16S and 23S rRNA genes. In the final strain, carrying no intact rRNA operon on the chromosome, rRNA molecules were expressed from a multicopy plasmid containing a single rRNA operon (prrn). Characterization of these rrn deletion strains revealed that deletion of two operons was required to observe a reduction in the growth rate and rRNA/protein ratio. When the number of deletions was extended from three to six, the decrease in the growth rate was slightly more than the decrease in the rRNA/protein ratio, suggesting that ribosome efficiency was reduced. This reduction was most pronounced in the Delta7 prrn strain, in which the growth rate, unlike the rRNA/protein ratio, was not completely restored to wild-type levels by a cloned rRNA operon. The decreases in growth rate and rRNA/protein ratio were surprisingly moderate in the rrn deletion strains; the presence of even a single operon on the chromosome was able to produce as much as 56% of wild-type levels of rRNA. We discuss possible applications of these strains in rRNA studies.     

Nucleotide sequence of the single ribosomal RNA operon of pea chloroplast DNA

The nucleotide sequence of an 8 kbp region of pea ( Pisum sativum L.) chloroplast DNA containing the rRNA operon and putative promoter sites has been determined and compared to the corresponding sequences from maize, tobacco and the liverwort Marchantia polymorpha . The chloroplast DNA species of all vascular plants investigated, with the exception of a few legumes including pea, and of Marchantia contain an inverted repeat with an rRNA operon. The pea rRNA operon is the first sequenced rRNA operon from a plant with only one copy of the rRNA genes per molecule of chloroplast DNA. The organization of the operon is the same as for maize, tobacco and Marchantia . i.e. tRNA-Val gene/16S rRNA gene/spacer with intron-containing genes for tRNA-Ile and tRNA-Ala/23S rRNA gene/4.5S rRNA gene/5S rRNA gene. Current evidence suggests that the tRNA-Val gene may not be contranscribed with the other genes. For pea 16S, 23S, 4.5S and 5S rRNA have 1488, 2813, 105 and 121 nucleotides, respectively. The homologies of the entire operon (the tRNA-Val gene - 5S rRNA region) to those from tobacco, maize and Marchantia are 88, 82 and 79%, respectively. The corresponding homologies for tobacco/maize, tobacco/ Marchantia and maize/ Marchantia have similar values. The 16S and 23S rRNA genes from pea are more than 90% homologous to those from the 3 other species. We conclude that the fact that pea only has one set of rRNA genes per molecule of chloroplast DNA is apparently not correlated with any significant difference between the pea operon and the rRNA operons from tobacco, maize and Marchantia .     

Introducing mutations into a chromosomal rRNA gene using a genetically modified eubacterial host with a single rRNA operon

Gene-inactivation techniques were employed to construct a eubacterial organism harbouring a single functional rRNA operon. This mutant of Mycobacterium smegmatis permits replacement of the single remaining rRNA operon with a homologous fragment from a vector-borne gene. By homologous recombination with the chromosome a plasmid-borne rDNA segment with resistance markers substitutes for the corresponding region of the chromosomal rRNA operon, resulting in a homogeneous population of mutated ribosomes in the cell. As a first result we demonstrate that the single allelic knock-out strain allows for isolation of rRNA mutants with a drug-resistant phenotype, circumventing the problem of recessivity which prohibits the isolation of such mutants in organisms with multiple rRNA operons. Subsequently, by allelic exchange experiments, it was demonstrated that the rRNA mutation found indeed confers drug resistance in vivo. This system provides intriguing potential for the study of the structure and function of ribosomal RNAs.     

Distinct Types of rRNA Operons Exist in the Genome of the Actinomycete Thermomonospora chromogena and Evidence for Horizontal Transfer of an Entire rRNA Operon

We describe here the presence of two distinct types of rRNA operons in the genome of a thermophilic actinomycete Thermomonospora chromogena. The genome of T. chromogena contains six rRNA operons (rrn), of which four complete and two incomplete ones were cloned and sequenced. Comparative analysis revealed that the operon rrnB exhibits high levels of sequence variations to the other five nearly identical ones throughout the entire length of the operon. The coding sequences for the 16S and 23S rRNA genes differ by approximately 6 and 10%, respectively, between the two types of operons. Normal functionality of rrnB is concluded on the basis of the nonrandom distribution of nucleotide substitutions, the presence of compensating nucleotide covariations, the preservation of secondary and tertiary rRNA structures, and the detection of correctly processed rRNAs in the cell. Comparative sequence analysis also revealed a close evolutionary relationship between rrnB operon of T. chromogena and rrnA operon of another thermophilic actinomycete Thermobispora bispora. We propose that T. chromogena acquired rrnB operon from T. bispora or a related organism via horizontal gene transfer.     

cpcHID操纵子序列用于钝顶节旋藻品系分类与鉴定的研究

克隆并测定7株钝顶节旋藻品系的cpcHID操纵子序列,以及16SrRNA和16S-23SrRNA转录单元内间隔区(ITS)序列,进一步通过生物信息学和分子系统学等研究发现:(1)7株品系的cpcHID序列,以及16SrRNA和ITS序列具有很高的相似性。(2)基于7株品系cpcHID序列的GC含量绝对偏差平均值、碱基变异率和遗传距离系数普遍比基于16SrRNA和ITS序列的大。(3)基于cpcHID序列的分类结果与基于16SrRNA和ITS序列的十分相近。因此,cpcHID可作为节旋藻等蓝细菌分类与鉴定的一种新的分子标记,特别是以其丰富的信息量而在品系水平的分类鉴定中占有优势。     

Evaluation of the rrn operon copy number in Bifidobacterium using real-time PCR

AIMS: A real-time PCR-based method was developed to evaluate the Bifidobacterium rRNA operon copy number. As a result of their repetitive nature, rRNA operons are very suitable targets for chromosomal integration of heterologous genes. METHODS AND RESULTS: The rrn operon multiplicity per chromosome was determined by real-time PCR quantification of the 16S rRNA amplicons obtained from genomic DNA. The values obtained in several bifidobacterial strains of human origin ranged from 1 to 5. The reliability of the method developed was confirmed by Southern hybridization technique. CONCLUSIONS: In the Bifidobacterium genus the rrn operon copies showed variability at species and strain level. The identification of Bifidobacterium strains with high rRNA multiplicity allowed the selection of potential hosts for chromosomal integration. SIGNIFICANCE AND IMPACT OF THE STUDY: The methodology here proposed represents a rapid, reliable and sensitive new tool for the quantification of rrn operon copy number in bacteria.     

Replication origin region of Bacillus subtilis chromosome contains two rRNA operons.

The first replicating DNA fragment (BamHI-7) of the Bacillus subtilis chromosome contains two promoters for a rRNA operon. A map of restriction enzyme cleavage sites of the region of replication origin suggests the presence of a second rRNA operon in this region. Hybridization of rRNA genes (rDNA) with DNA fragments derived from the origin region by treatment with various enzymes clearly revealed two rRNA operons in this region, one at the B7-B3 junction and the other at the B5-B6 junction. The restriction enzyme cleavage sites surrounding the rRNA operons show that the operon at the B5-B6 junction corresponds to the rrnA operon. A novel operon at the B7-B3 junction was termed rrnO. Transformation by density-labeled fragments of the origin region showed that the first replicating marker, guaA, is located in the B3 fragment. From these results, a map was constructed for the first time to correlate the genetic markers with the physical structure of the replication origin region of the B. subtilis chromosome. The role of the rrnO operon in regulating the initiation of chromosomal replication is discussed, based on the fact that the promoter of the rrnO operon suppresses the replication of the plasmid carrying the promoter.     

Nucleotide sequence of the Bacillus subtilis ribosomal RNA operon, rrnB

The primary sequence of DNA covering a complete ribosomal RNA (rRNA) operon from Bacillus subtilis, designated rrnB has been elucidated. Following a set of tandem promoters, rrnB encodes: (i) a 16S and a 23S rRNA determinant with no tRNA spacer region in between; (ii) a 5S rRNA determinant; and (iii) 21 contiguous tRNA species; before (iv) two characteristic terminator hairpins are found. More than 7 kb are included within this operon, which maps between aroG and thr5 on the B. subtilis chromosome. This represents the first report of the entire sequence of an rRNA operon from B. subtilis or from any Gram-positive organism.     

Isolation of cDNA clones for the human transferrin receptor.

A cDNA clone bank containing 30 000 clones was constructed from sucrose gradient-fractionated mRNA from human placenta. mRNA coding for transferrin receptor (TR) was enriched by polysome immuno-adsorbed chromatography with monospecific rabbit IgG and protein-A Sepharose. The library was screened for hybridisation to 32P-labelled cDNA synthesised from immunoselected TR mRNA and from poly(A)+ RNA of the polysome fraction that failed to bind to protein-A Sepharose. Plasmids isolated from colonies showing hybridisation only to the probe made from immunoselected mRNA were then subjected to hybrid selection. Two clones, pTR-48 and pTR-67, were able to hybridise the mRNA coding for the TR.     

Mycoplasmas (Mollicutes) have a low number of rRNA genes.

DNA from Mycoplasma, Ureaplasma, Acholeplasma, and Spiroplasma species digested by restriction endonucleases was hybridized with probes consisting of portions of the rrnB rRNA operon of Escherichia coli and the rRNA operon of Mycoplasma capricolum. The results indicate the presence of only one or two sets of rRNA genes in the genome of Mollicutes linked in the procaryotic fashion, 16S-23S-5S.     

Variable numbers of rRNA gene operons in Bacillus cereus strains

Abstract Ribosornal RNA operon organisation was analysed in two Bacillus cereus strains of different chromosome size, ATCC 10987 (5.4 Mb) and F0837/76 (2.4 Mb). We estimated that there were twelve and nine copies of the rRNA operons in these two strains, respectively. In B. cereus ATCC 10987 six rRNA operons were less than 10 kb apart, while in B. cereus F0837/76 four rRNA operons were similarly clustered. The origin of replication was located in the vicinity of a rRNA operon in both strains.     

Drivers of epsilonproteobacterial community composition in sulfidic caves and springs

Epsilonproteobacteria are widely distributed in marine, freshwater, and terrestrial environments, although most well-studied groups are from hydrothermal vents and the human intestinal tract. The environmental variables that control epsilonproteobacterial communities in sulfidic terrestrial environments, however, are poorly understood. Here, the environmental variables that influence epsilonproteobacterial community composition in geographically separated sulfidic caves and springs were determined by coarse and fine-scale approaches: denaturing gradient gel electrophoresis profiling of 23S rRNA PCR amplicons and clone library sequencing of the 16S-ITS-23S rRNA operon. Sequences retrieved from this study were not closely related to cultured representatives, indicating that existing culture collections do not adequately capture the diversity of terrestrial Epsilonproteobacteria. Comparisons of 16S-ITS-23S rRNA operon sequences from four sites revealed that some distant communities (> 8000 km) share closely related populations of Epsilonproteobacteria, while other sites have nearly clonal and phylogenetically distinct populations. Statistical evaluations of sequence data reveal that multiple environmental variables (e.g. temperature, pH, salinity, dissolved oxygen, and bicarbonate concentrations) influence Epsilonproteobacteria community composition. Locations with clonal populations tended to be from higher temperatures and intermediate dissolved oxygen concentrations. rRNA operon sequences outside of the 16S rRNA gene may be critical to recognizing environmental drivers of epsilonproteobacterial community composition.     

A coarse-grained biophysical model of E. coli and its application to perturbation of the rRNA operon copy number

We propose a biophysical model of Escherichia coli that predicts growth rate and an effective cellular composition from an effective, coarse-grained representation of its genome. We assume that E. coli is in a state of balanced exponential steady-state growth, growing in a temporally and spatially constant environment, rich in resources. We apply this model to a series of past measurements, where the growth rate and rRNA-to-protein ratio have been measured for seven E. coli strains with an rRNA operon copy number ranging from one to seven (the wild-type copy number). These experiments show that growth rate markedly decreases for strains with fewer than six copies. Using the model, we were able to reproduce these measurements. We show that the model that best fits these data suggests that the volume fraction of macromolecules inside E. coli is not fixed when the rRNA operon copy number is varied. Moreover, the model predicts that increasing the copy number beyond seven results in a cytoplasm densely packed with ribosomes and proteins. Assuming that under such overcrowded conditions prolonged diffusion times tend to weaken binding affinities, the model predicts that growth rate will not increase substantially beyond the wild-type growth rate, as indicated by other experiments. Our model therefore suggests that changing the rRNA operon copy number of wild-type E. coli cells growing in a constant rich environment does not substantially increase their growth rate. Other observations regarding strains with an altered rRNA operon copy number, such as nucleoid compaction and the rRNA operon feedback response, appear to be qualitatively consistent with this model. In addition, we discuss possible design principles suggested by the model and propose further experiments to test its validity.     

Nucleotide sequence of a 'truncated rRNA operon' of the Euglena gracilis chloroplast genome.

An extra 16S rRNA gene (s-16S rDNA) from the Euglena gracilis chloroplast genome and several hundred positions of its flanking regions have been sequenced. The structural part has 1486 positions and is to 98% homologous in its sequence with the 16S rRNA gene in functional chloroplast rRNA operons. Sequences of about 200 positions upstream and 15 positions downstream of the structural part of the s-16S rRNA gene region are highly homologous with corresponding parts in the functional operon. Neither tRNA genes (A1a, I1e) nor parts of the 23S and 5S rRNA genes are found within 557 positions after the 3' end of the s-16S rRNA gene, i.e., the 330 bp homology, observed in electron microscopic studies of heteroduplexes (4), between the s-16S rDNA downstream region and the 6.2 kb repeated segment containing the functional rRNA operon, must be due to a DNA stretch in the interoperon spacer. A structural model of the "truncated rRNA operon" is presented. Results from S-1 endonuclease analysis suggest that the s-16S rDNA region is probably not transcribed into stable s-16S rRNA.