Two classes of DNA elements interrupt a fraction of the rRNA repeats of Bombyx mon. We have analyzed by genomic blotting and sequence analysis one class of these elements which we have named R2. These elements occupy approximately 9% of the rDNA units of B. mon and appear to be homologous to the type II rDNA insertions detected in Drosophila melanogaster. Approximately 25 copies of R2 exist within the B. mori genome, of which at least 20 are located at a precise location within otherwise typical rDNA units. Nucleotide sequence analysis has revealed that the 4.2-kilobase-pair R2 element has a single large open reading frame, occupying over 82% of the total length of the element. The central region of this 1,151-amino-acid open reading frame shows homology to the reverse transcriptase enzymes found in retroviruses and certain transposable elements. Amino acid homology of this region is highest to the mobile line 1 elements of mammals, followed by the mitochondrial type II introns of fungi, and the pol gene of retroviruses. Less homology exists with transposable elements of D. melanogaster and Saccharomyces cerevisiae. Two additional regions of sequence homology between LI and R2 elements were also found outside the reverse transcriptase region. We suggest that the R2 elements are retrotransposons that are site specific in their insertion into the genome. Such mobility would enable these elements to occupy a small fraction of the rDNA units of B. mori despite their continual elimination from the rDNA locus by sequence turnover.A fraction of the 28S ribosomal genes in several insect species is interrupted by segments of non-rDNA approximately 5 kilobases (kb) in length (see the review by Beckingham [3]). Based on the nucleotide sequences of their junction regions, these insertions have been divided into two classes. Type I elements have been reported in Drosophila virilis (34), D. melanogaster (7, 37), Calliphora erythrocephala (43), and Bombyx mori (12,16). They interrupt the 28S rRNA gene at a location approximately two-thirds of the distance from the 5' end of the gene and are flanked by a 14-base-pair (bp) duplication of rDNA sequences. Type II elements have only been reported in D. melanogaster (7,37) and B. mori (12,16). They interrupt the 28S gene 75 bp upstream of the type I elements and do not contain flanking duplications of rDNA sequences. In D. melanogaster, rDNA units containing either of these elements are transcribed at a significantly lower level than are the remaining rDNA units (20,23,28). To be consistent with the singleletter nomenclature frequently used to describe repetitive or transposable DNA elements and to avoid possible confusion with the type I and II intron sequences of fungal mitochondrial DNAs, we suggest that these elements be referred to as the Ri (type I) and R2 (type II) elements (R referring to rDNA units).Disruption of the large rRNA genes has also been detected in noninsect species. In Ascaris lumbricoides, a 4.5-kb element interrupts a small fraction of the 26S rRNA genes at ...

During isocapnic hypoxia in healthy persons, cerebral oxygenation as estimated by near infrared spectroscopy precisely tracks changes in measured S(jv)O2 within individuals, but the relation exhibits a wide range of slopes and intercepts. Therefore the clinical utility of the device is limited to situations in which tracking trends in cerebral oxygenation would be acceptable.

No abstract

No abstract