一类新内含子的研究状况

在真核细胞基因组中发现一类新内含子——AT-AC内含子, 除结构特征与普遍存在的核mRNA前体内含子有明显不同外,剪接机制上也存在一定差异.文章介绍了该内含子的分布,结构特征及剪接机理,并与主内含子作了相应比较.     

Sequence and properties of the human KB cell and mouse L cell D-loop regions of mitochondrial DNA.

The sequences of the displacement-loop (D-loop) regions of mitochondrial DNA (mtDNA) from mouse L cells and human KB cells have been determined and provide physical maps to aid in the identification of sequences involved in the regulation of replication and expression of mammalian mtDNA. Both D-loop regions are bounded by the genes for tRNAPhe and tRNAPro. This region contains the most highly divergent sequences in mtDNA with the exceptions of three small conserved sequence blocks near the 5' ends of D-loop strands, a 225 nucleotide conserved sequence block in the center of the D-loop strand template region, and a short sequence associated with the 3' ends of D-loop strands. A sequence similar to that associated with the 3' termini of D-loop strands overlaps one of the conserved sequence blocks near the 5' ends of D-loop strands. The large, central conserved sequence probably does not code for a protein since no open reading frames are discretely conserved. Numerous symmetric sequences and potential secondary structures exist in these sequences, but none appear to be clearly conserved between species.     

The primary structure of the Saccharomyces cerevisiae gene for alcohol dehydrogenase

The DNA sequence of the gene for the fermentative yeast alcohol dehydrogenase has been determined. The structural gene contains no introns. The amino acid sequence of the protein as determined from the nucleotide sequence disagrees with the published alcohol dehydrogenase isozyme I (ADH-I) sequence for 5 of the 347 amino acid residues. At least one, and perhaps as many as four, of these differences is probably due to ADH-I protein heterogeneity in different yeast strains and not to sequencing errors. S1 nuclease was used to map the 5' and 3' ends of the ADH-I mRNA. There are two discrete, mature 5' ends of the mRNA, mapping 27 and 37 nucleotides upstream of the translation initiating ATG. These two equally prevalent termini are 101 and 91 nucleotides, respectively, downstream from a TATAAA sequence. Analysis of the 3' end of ADH-I mRNA disclosed two minor ends upstream of the major poly(A) addition site. These three ends map 24, 67, and 83 nucleotides, respectively, downstream from the translation-terminating TAA triplet. The sequence AA-TAAG is found 28 to 34 nucleotides upstream of each ADH-I mRNA poly(A) addition site. Sequence comparisons of these three 3' ends with those for four other yeast mRNAs yielded a 13-nucleotide consensus sequence to which TAAATAAGA is central. All of the known yeast poly(A) addition sites map at or near the A residue of a CTA site 25 to 40 nucleotides downstream from this consensus octamer.     

Transfer RNA genes in Drosophila mitochondrial DNA: related 5'' flanking sequences and comparisons to mammalian mitochondrial tRNA genes.

Genes for tRNAgly and tRNAserUCN have been identified within sequences of mtDNA of Drosophila yakuba. The tRNAgly gene lies between the genes for cytochrome c oxidase subunit III and URF3, and all three of these genes are contained in the same strand of the mtDNA molecule. The tRNAserUCN gene is adjacent to the URF1 gene. These genes are contained in opposite strands of the mtDNA molecule and their 3' ends overlap. The structures of the tRNAgly and tRNAserUCN genes, and of the four tRNA genes of D. yakuba mtDNA reported earlier (tRNAile, tRNAgln, tRNAf-met and tRNAval) are compared to each other, to non-organelle tRNAs, and to corresponding mammalian mitochondrial tRNA genes. Within 19 nucleotides upstream from the 5' terminal nucleotide of each of the Drosophila mitochondrial tRNAgly, tRNAserUCN, tRNAile, tRNAgln and tRNAf-met genes occurs the sequence 5'TTTATTAT, or a sequence differing from it by one nucleotide substitution. Upstream from this octanucleotide sequence, and separated from it by 3, 4 and 11 nucleotides, respectively, in the 5' flanking regions of the tRNAile, tRNAserUCN and tRNAgly genes occurs the sequence 5'GATGAG.     

Linearization and transposition of circular molecules of insertion sequence IS3.

IS3 transposase has been shown to promote production of characteristic circular and linear IS3 molecules from the IS3-carrying plasmid; IS3 circles have the entire IS3 sequence with terminal inverted repeats, IRL and IRR, which are separated by a three base-pair sequence originally flanking either end in the parental plasmid, whereas linear IS3 molecules have three nucleotide overhangs at their 5' ends. Here, we showed that a plasmid carrying an IS3 derivative, which is flanked by different sequences at both ends, generated IS3 circles and linear IS3 molecules owing to the action of transposase. Cloning and sequencing analyses of the linear molecules showed that each had the same 5'-protruding three nucleotide overhanging sequences at both ends, suggesting that the linear molecules were not generated from the parental plasmid by the two double-strand breaks at both end regions of IS3. The plasmid carrying IS3 with a two base-pair mutation in the terminal dinucleotide, which would be required for transposase to cleave the 3' end of IS3, could still generate linear molecules as well as circles. Plasmids bearing an IS3 circle were cleaved by transposase and gave linear molecules with the same 5'-protruding three nucleotide overhanging sequences. These show that the linear molecules are generated from IS3 circles via a double-strand break at the three base-pair intervening sequence. Plasmids carrying an IS3 circle with the two base-pair end mutation still were cleaved by transposase, though with reduced efficiencies, suggesting that IS3 transposase has the ability to cleave not only the 3' end of IS3, but a site three nucleotides from the 5' end of IS3. IS3 circles also were shown to transpose to the target plasmids. The end mutation almost completely inhibited this transposition, showing that the terminal dinucleotides are important for the transfer of the 3' end of IS3 to the target as well as for the end cleavage.     

Natural selection affects frequencies of AG and GT dinucleotides at the 5' and 3' ends of exons

GT and AG, located at the 5' and 3' ends of introns, are important for correct splicing. It is anticipated that natural selection decreases frequency of AG and GT near the 5' and 3' ends of exons, preventing appearance of cryptic splicing sites. The data presented in this article support the expectation.     

Complete nucleotide sequence of a functional HLA-DP beta gene and the region between the DP beta 1 and DP alpha 1 genes: comparison of the 5'' ends of HLA class II genes.

The complete nucleotide sequence of an HLA-DP beta 1 gene and part of the adjacent DP alpha 1 gene, up to and including the signal sequence exon, were determined. The sequence of the DP beta 1 gene identified it as the DPw4 allele. The six exons of the DP beta 1 gene spanned over 11,000 bp of sequence. The arrangement of the gene was broadly analogous to genes of other class II beta chains. The beta 1 exon was flanked by introns of over 4 kb. Comparisons with published sequences of cDNA clones indicated that an alternative splice junction, at the 3' end of the gene, is used in at least one allele. Variation in choice of splice junction indicates an additional mechanism for allelic variation in class II genes. The sequence also indicated that the DP beta 1 and DP alpha 1 genes are separated by only 2 kb at their 5' ends. Comparison of the 5' ends of the DP alpha 1 and beta 1 genes with other class II sequences, including the DZ alpha gene, showed conservation of several blocks of sequences thought to be involved in control of expression. Some areas of the introns were partially conserved in the DQ beta gene, and several other intron sequences were homologous to sequences found in other unrelated genes.     

Characterization of DNA end joining in a mammalian cell nuclear extract: junction formation is accompanied by nucleotide loss, which is limited and uniform but not site specific.

Mammalian cells have a marked capacity to repair double-strand breaks in DNA, but the molecular and biochemical mechanisms underlying this process are largely unknown. A previous report has described an activity from mammalian cell nuclei that is capable of multimerizing blunt-ended DNA substrates (R. Fishel, M.K. Derbyshire, S.P. Moore, and C.S.H. Young, Biochimie 73:257-267, 1991). In this report, we show that nuclear extracts from HeLa cells contain activities which preferentially join linear plasmid substrates in either a head-to-head or tail-to-tail configuration, that the joining reaction is covalent, and that the joining is accompanied by loss of sequence at the junction. Sequencing revealed that there was a loss of a uniform number of nucleotides from junctions formed from any one type of substrate. The loss was not determined by any simple site-specific mechanism, but the number of nucleotides lost was affected by the precise terminal sequence. There was no major effect on the efficiency or outcome of the joining reaction with substrates containing blunt ends or 3' or 5' protruding ends. Using a pair of plasmid molecules with distinguishable restriction enzyme sites, we also observed that blunt-ended DNA substrates could join with those containing protruding 3' ends. As with the junctions formed between molecules with identical ends, there was uniform loss of nucleotides. Taken together, the data are consistent with two models for the joining reaction in which molecules are aligned either throughout most of their length or by using small sequence homologies located toward their ends. Although either model can explain the preferential formation of head-to-head and tail-to-tail products, the latter predicts the precise lossof nucleotides observed. These activities are found in all cell lines examined so far and most likely represent an important repair activity of the mammalian cell.