RESUMO
Eukaryotic ribosomal frameshift signals generally contain two elements, a heptanucleotide slippery sequence (XXXYYYN) and an RNA secondary structure, often an RNA pseudoknot, located downstream. Frameshifting takes place at the slippery sequence by simultaneous slippage of two ribosome-bound tRNAs. All of the tRNAs that are predicted to decode frameshift sites in the ribosomal A-site (XXXYYYN) possess a hypermodified base in the anticodon-loop and it is conceivable that these modifications play a role in the frameshift process. To test this, we expressed slippery sequence variants of the coronavirus IBV frameshift signal in strains of Escherichia coli unable to modify fully either tRNA(Lys) or tRNA(Asn). At the slippery sequences UUUAAAC and UUUAAAU (underlined codon decoded by tRNA(Asn), anticodon 5' QUU 3'), frameshifting was very inefficient (2 to 3%) and in strains deficient in the biosynthesis of Q base, was increased (AAU) or decreased (AAC) only two-fold. In E. coli, therefore, hypomodification of tRNA(Asn) had little effect on frameshifting. The situation with the efficient slippery sequences UUUAAAA (15%) and UUUAAAG (40%) (underlined codon decoded by tRNA(Lys), anticodon 5' mnm5s2UUU 3') was more complex, since the wobble base of tRNA(Lys) is modified at two positions. Of four available mutants, only trmE (s2UUU) had a marked influence on frameshifting, increasing the efficiency of the process at the slippery sequence UUUAAAA. No effect on frameshifting was seen in trmC1 (cmnm5s2UUU) or trmC2 (nm5s2UUU) strains and only a very small reduction (at UUUAAAG) was observed in an asuE (mnm5UUU) strain. The slipperiness of tRNA(Lys), therefore, cannot be ascribed to a single modification site on the base. However, the data support a role for the amino group of the mnm5 substitution in shaping the anticodon structure. Whether these conclusions can be extended to eukaryotic translation systems is uncertain. Although E. coli ribosomes changed frame at the IBV signal (UUUAAAG) with an efficiency similar to that measured in reticulocyte lysates (40%), there were important qualitative differences. Frameshifting of prokaryotic ribosomes was pseudoknot-independent (although secondary structure dependent) and appeared to require slippage of only a single tRNA.
Assuntos
Anticódon/genética , Escherichia coli/genética , Mudança da Fase de Leitura do Gene Ribossômico/genética , Vírus da Bronquite Infecciosa/genética , Sequência de Bases , Dados de Sequência Molecular , Conformação de Ácido Nucleico , Plasmídeos/genética , Mutação Puntual , RNA de Transferência de Asparagina/genética , RNA de Transferência de Lisina/genética , RNA Viral/química , RNA Viral/genéticaRESUMO
UNLABELLED: The positions of the 18S-5.8S-26S and 5S rRNA genes have been physically mapped on the chromosomes of diploid, tetraploid, and hexaploid Festuca species by in situ hybridization. The number and position of the rDNA sites in the species were compared. The results confirm some of the earlier phylogenetic studies of these species but suggest that some structural rearrangements have occurred and that sites have been lost during polyploidization. KEYWORDS: Festuca, in situ hybridization, phylogeny, physical mapping, rDNA.
RESUMO
The position of the 18S-5.8S-26S and 5S rRNA genes have been physically mapped on the chromosomes of seven Lolium taxa. 18S-5.8S-26S sites were seen on two pairs of chromosomes in the inbreeding taxa. In the outbreeding taxa six sites were found in the L. multiflorum, seven in L. perenne and nine in L. rigidum var. rigidum. Two 5S sites were found in each of the taxa. In the inbreeders, the 5S sites were found adjacent to the 18S-5.8S-26S sites on chromosome 2. In L. multifiorum and L.perenne the 5S sites were on the short arm of chromosome 3. However, in L. rigidum var. rigidum the 5S rDNA site was found in either of the two positions.
Assuntos
DNA Ribossômico/genética , Lolium/genética , Mapeamento Cromossômico , Cromossomos , Sondas de DNA/genética , Corantes Fluorescentes , Genes de Plantas/genética , Genótipo , Hibridização in Situ Fluorescente , Microscopia de Fluorescência , RNA Ribossômico 5S/genéticaRESUMO
Fluorescence in situ hybridization (FISH) using total genomic DNA from putative diploid progenitors was used to confirm the presence of the A and C genomes in Avena maroccana. These results confirm cytological data that intergenomic translocations are present in A. maroccana.
Assuntos
Grão Comestível/genética , Genoma , Hibridização in Situ Fluorescente , Translocação Genética , Evolução Biológica , Sondas de DNA , PloidiasRESUMO
Genomic in situ hybridization (GISH) was used to identify Festuca chromatin in mitotic chromosomes of Lolium multiflorum (Lm) × Festuca pratensis (Fp) hybrids and hybrid derivatives. In two inverse autoallotriploids LmLmFp and LmFpFp, in situ hybridization was able to discriminate between the Lolium and Festuca chromosomes. In a third triploid hybrid produced by crossing an amphiploid of L. multiflorum × F. pratensis (2n=4x=28) with L. multiflorum (2n=2x=14), the technique identified chromosomes with interspecific recombination. Also, in an introgressed line of L. multiflorum which was homozygous for the recessive sid (senescence induced degradation) allele from F. pratensis, a pair of chromosome segments carrying the sid gene could be identified, indicating the suitability of GISH in showing the presence and location of introgressed genes. By screening backcross progeny for the presence of critical alien segments and the absence of other segments the reconstitution of the genome of the recipient species can be accelerated.
RESUMO
The mouse conceptus is generally held to be radially symmetrical about its embryonic-abembryonic axis from the blastocyst stage until the primitive streak appears at the beginning of gastrulation. However, this notion has been challenged by recent observations on conceptuses sectioned in utero which suggest that the blastocyst is already bilaterally symmetrical when it begins to implant. Accordingly, the blastocyst has been assigned an anterior-posterior axis which appears to persist through gastrulation and is claimed to coincide with the anterior-posterior axis of the future fetus in both orientation and polarity. In the present investigation the relationship between these two axes was examined in conceptuses dissected from the uterus early in gastrulation so that it could be determined more accurately than is possible in situ. The anterior-posterior axis of the conceptus and nascent fetus were found to be either parallel or antiparallel to each other, suggesting that while the orientation of the fetal axis may be specified at the blastocyst stage its polarity is not.
