Nucleotide sequence of the 3''-terminal region of rat 18S ribosomal DNA

Summary The 3-terminal 230 base-pairs (bp) of the gene for 18S rRNA and 40 bp of the adjoining spacer have been sequenced for the Sprague-Dawley rat. This mammalian sequence has been compared with the known sequences of yeast, fruit fly, silkworm, and frog. This study has shown that the nucleotide-sequence differences between rat and frog are the smallest among the five species, probably reflecting their evolutionary closeness and longer maturation time compared to the others. There is little similarity in the nucleotide sequences of the transcribed spacer regions of the five species compared.     

Nucleotide sequence of the 5'-terminal region of rat 18S ribosomal DNA

The 5'-terminal 597 base-pairs (bp) of the Sprague-Dawley rat 18S ribosomal RNA gee and 10 bp of the adjoining transcribed spacer have been sequenced. Previously sequenced 10 large oligonucleotides of rat 18S RNA were located in this region. This mammalian sequence has been compared with the known sequences of yeast and frog 18S rDNA's. The analysis indicates that 534 bp of the 597 bp (89%) are conserved between rat and frog sequences but only 75% of the nucleotides are conserved between rat and yeast in this region. Two large and two small sections have been identified where insertions have been introduced during evolution. Of these 58 bp long inserted sections of the rat rDNA sequence, 50 bp (86%) were G-C base-pairs.     

Turnover of ribosomal 28S and 18S rRNA during rat liver regeneration

The turnover of 28S and 18S rRNA was studied in the course of 12 d after partial hepatectomy, including the proliferative (1st to 5th d) and post-proliferative (6th to 12th d) phases of liver regeneration. Turnover data, as the day-to-day rates of synthesis and degradation of 28S and 18S rRNA, were obtained by employing a suitable experimental procedure for the estimation of the increase of the amount of rRNA in the regenerating liver. It was found that 28S and 18S rRNA are accumulated into the cytoplasm and degraded at identical rates both in the proliferative and post proliferative phases. The turnover of both rRNA moieties is markedly slower during the first 3 d of liver regeneration.     

Nucleotide sequence of a mosquito 18S ribosomal RNA gene.

We have sequenced an 18S ribosomal RNA gene from the mosquito, Aedes albopictus. Computer alignment of the 1950 nucleotide coding region (56% A + T) with 18S rRNA sequences from two insect and three vertebrate species revealed greater sequence divergence among the insects than among the vertebrates. Sequence alignments showed that variable region V4, which has been considered to be the most poorly conserved domain in the 18S rRNA gene, was better conserved among insects and vertebrates than was the V6 domain.     

Nucleotide sequence relationships between vertebrate 5.8 S ribosomal RNAs.

Nucleotide sequences of the 5.8 S ribosomal RNAs from HeLa cells, Xenopus laevis and chick embryo fibroblasts were compared. Xenopus laevis 5.8 S RNA differs from that of HeLa cells in four internal positions and at the 3' end of the molecule. Chick 5.8 S RNA differs from that of HeLa cells in two positions. Six out of the seven interspecies differences are due to base substitutions. The other difference is due to the presence of an extra nucleotide, internally located, within the Xenopus 5.8 S sequence.     

The longest 18S ribosomal RNA ever known. Nucleotide sequence and presumed secondary structure of the 18S rRNA of the pea aphid, Acyrthosiphon pisum.

An EMBL4 recombinant phage which encodes one of the full length of the aphid ribosomal DNA has been isolated from the aphid genomic library. Determination of the complete nucleotide sequence of the aphid 18S rRNA gene revealed that it is 2469 bp with a G + C content of 59%. The aphid 18S rRNA gene studied here is the longest and has the highest G + C content among the 18S rRNA genes examined so far. Evidence provided by the S1 nuclease assay suggests that the aphid 18S rRNA gene examined in this study is not a pseudogene containing an insertion sequence. Based on the nucleotide sequence of the 18S rRNA gene, we constructed a presumed secondary-structure model of the aphid 18S rRNA. In the aphid 18S rRNA, the eucaryote-specific E21 and 41 region are supposed to be longer and more complex than the counterparts of other 18S rRNA.     

Nucleotide sequence encoding the 5'' end of Xenopus laevis 18S rRNA.

We have sequenced a region of cloned Xenopus laevis ribosomal DNA encompassing the last 24 nucleotides of the external transcribed spacer and the first 275 nucleotides of the 18S gene. The start of the 18S gene was identified by correlating the results obtained from RNA hybridization and fingerprinting with the DNA sequence. This 5' region of 18S rRNA contains five 2'-O-methyl groups and at least six pseudouridine residues. Several of these modified nucleotides are clustered into a relatively short region from nucleotides 99-124. Nucleotides 227-250 constitute a distinctive sequence of 24 consecutive G and C residues. Comparison with the first 160 nucleotides of a yeast 18S gene (25) reveals three blocks of high sequence homology separated by two short tracts where homology is low or absent. The external transcribed spacer sequences diverge widely from within a few nucleotides of the start of the 18S gene.     

Nucleotide sequence and evolution of the 18S ribosomal RNA gene in maize mitochondria.

The nucleotide sequence of the gene coding for the 18S ribosomal RNA of maize mitochondria has been determined and a model for the secondary structure is proposed. Dot matrix analysis has been used to compare the extent and distribution of sequence similarities of the entire maize mitochondrial 18S rRNA sequence with that of 15 other small subunit rRNA sequences. The mitochondrial gene shows great similarity to the eubacterial sequences and to the maize chloroplast, and less similarity to mitochondrial rRNA genes in animals and fungi. We propose that this similarity is due to a slow rate of nucleotide divergence in plant mtDNA compared to the mtDNA of animals. Sequence comparisons indicate that the evolution of the maize mitochondrial 18S, chloroplast 16S and nuclear 17S ribosomal genes have been essentially independent, in spite of evidence for DNA transfer between organelles and the nucleus.     

Human 18 S ribosomal RNA sequence inferred from DNA sequence. Variations in 18 S sequences and secondary modification patterns between vertebrates.

We have determined the DNA sequences encoding 18 S ribosomal RNA in man and in the frog, Xenopus borealis. We have also corrected the Xenopus laevis 18 S sequence: an A residue follows G-684 in the sequence. These and other available data provide a number of representative examples of variation in primary structure and secondary modification of 18 S ribosomal RNA between different groups of vertebrates. First, Xenopus laevis and Xenopus borealis 18 S ribosomal genes differ from each other by only two base substitutions, and we have found no evidence of intraspecies heterogeneity within the 18 S ribosomal DNA of Xenopus (in contrast to the Xenopus transcribed spacers). Second, the human 18 S sequence differs from that of Xenopus by approx. 6.5%. About 4% of the differences are single base changes; the remainder comprise insertions in the human sequence and other changes affecting several nucleotides. Most of these more extensive changes are clustered in a relatively short region between nucleotides 190 and 280 in the human sequence. Third, the human 18 S sequence differs from non-primate mammalian sequences by only about 1%. Fourth, nearly all of the 47 methyl groups in mammalian 18 S ribosomal RNA can be located in the sequence. The methyl group distribution corresponds closely to that in Xenopus, but there are several extra methyl groups in mammalian 18 S ribosomal RNA. Finally, minor revisions are made to the estimated numbers of pseudouridines in human and Xenopus 18 S ribosomal RNA.     

Nucleotide sequence of the 16S - 23S spacer region in an rRNA gene cluster from tobacco chloroplast DNA.

The nucleotide sequence of a spacer region between 16S and 23S rRNA genes from tobacco chloroplasts has been determined. The spacer region is 2080 bp long and encodes tRNAIle and tRNAAla genes which contain intervening sequences of 707 bp and 710 bp, respectively. Strong homology between the two intervening sequences is observed. These spacer tRNAs are synthesized as part of an 8.2 kb precursor molecule containing 16S and 23S rRNA sequences.