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271.
M.K. Tan 《Journal of molecular evolution》1997,44(6):637-645
Studies of the distribution of the three group I introns (intron A, intron T, and intron AT) in the 26S rDNA of Gaeumannomyces graminis had suggested that they were transferred to a common ancestor of G. graminis var. avenae and var. tritici after it had branched off from var. graminis. Intron AT and intron A exhibited vertical inheritance and coevolved in concert with their hosts. Intron loss could occur
after its acquisition. Loss of any one of the three introns could occur in var. tritici whereas only loss of intron T had been found in the majority of var. avenae isolates. The existence of isolates of var. tritici and var. avenae with three introns suggested that intron loss could be reversed by intron acquisition and that the whole process is a dynamic
one. This process of intron acquisition and intron loss reached different equilibrium points for different varieties and subgroups,
which explained the irregular distribution of these introns in G. graminis. Each of the three group I introns was more closely related to other intron sequences that share the same insertion point
in the 26S rDNA than to each other. These introns in distantly related organisms appeared to have a common ancestry. This
system had provided a good model for studies on both the lateral transfer and common ancestry of group I introns in the 26S
rRNA genes.
Received: 17 May 1996 / Accepted: 14 January 1997 相似文献
272.
273.
Mitochondrial introns in flowering plant genes are virtually all classified as members of the group II ribozyme family although certain structural features have degenerated to varying degrees over evolutionary time. We are interested in the impact that unconventional intron architecture might have on splicing biochemistry in vivo and we have focused in particular on intronic domains V and VI, which for self-splicing introns provide a key component of the catalytic core and the bulged branchpoint adenosine, respectively. Notably, the two transesterification steps in classical group II splicing are the same as for nuclear spliceosomal introns and release the intron as a lariat. Using RT-PCR and circularized RT-PCR, we had previously demonstrated that several wheat mitochondrial introns which lack a branchpoint adenosine have atypical splicing pathways, and we have now extended this analysis to the full set of wheat introns, namely six trans-splicing and sixteen cis-splicing ones. A number of introns are excised using non-lariat pathways and interestingly, we find that several introns which do have a conventional domain VI also use pathways that appear to exploit other internal or external nucleophiles, with the lariat form being relatively minor. Somewhat surprisingly, several introns with weakly-structured domain V/VI helices still exhibit classical lariat splicing, suggesting that accessory factors aid in restoring a splicing-competent conformation. Our observations illustrate that the loss of conventional group II features during evolution is correlated with altered splicing biochemistry in an intron-distinctive manner. 相似文献
274.
275.
Fei Yan Jiejun Peng Yuwen Lu Lin Lin Hongying Zheng Hairu Chen Jianping Chen Michael J. Adams 《Plant cell reports》2009,28(2):241-246
In Arabidopsis thaliana, Dicer-like protein 2 (DCL2) cleaves double-stranded virus RNA, playing an essential role in the RNA interference pathway.
Here, we describe three alternative splicing (AS) forms of AtDCL2: in one, both intron 8 and intron 10 are retained in the
mRNA, in second only intron 8 is retained and in the third no intron is retained, but there is a deletion of 56 nucleotides
at the end of exon 10. These splicing forms are present in stems and leaves at different development stages. AS was also detected
in DCL2 of Brassica rapa, where intron 9, but not intron 8 or intron 10, was retained suggesting that AS may be a common phenomenon in cruciferous
plant DCL2s. The retained introns and sequence deletions detected in AtDCL2 changed the reading frame and produced premature
terminal codons. The AS forms appeared to be substrates of nonsense-mediated decay of mRNA.
Fei Yan and Jiejun Peng contributed equally to this work. 相似文献
276.
Importin αs are import receptors for nuclear localization signal-containing proteins. Most animal importin αs assort into
α1, α2, and α3 groups. Studies in Drosophila melanogaster, Caenorhabditis elegans, and mouse suggest that the animal importin α gene family evolved from ancestral plant-like genes to serve paralog-specific
roles in gametogenesis. To explore this hypothesis we extended the phylogenetic analysis of the importin α gene family to
nonbilateral animals and investigated whether animal-like genes occur in premetazoan taxa. Maximum likelihood analysis suggests
that animal-like importin α genes occur in the Choanoflaggelate Monosiga brevicollis and the amoebozoan Dictyostelium; however, both of these results are caused by long-branch attraction effects. The absence of animal-like α genes in premetazoan
taxa is consistent with the hypothesis that they duplicated and then specialized to function in animal gametogenesis. The
gene structures of the importin αs provide insight into how the animal importin α gene family may have evolved from the most
likely ancestral gene. Interestingly, animal α1s are more similar to plant and fungal α1-like sequences than they are to animal
α2s or α3s. We show that animal α1 genes share most of their introns with plant α1-like genes, and α2s and α3s share many
more intron positions with each other than with the α1s. Together, phylogenetics and gene structure analysis suggests a parsimonious
path for the evolution of the mammalian importin α gene family from an ancestral α1-like progenitor. Finally, these results
establish a rational basis for a unified nomenclature of the importin α gene family. 相似文献
277.
278.
The intron positions of ten different protein families were examined to determine (the statistical likelihood of) whether
spliceosomal introns are the result of random insertion events into previously intronless genes, on the one hand, or the result
of random loss from common ancestral introns, on the other. The number of expected matches for the alternative scenarios was
calculated for a binomial distribution by considering currently observed introns relative to all possible locations for insertion
or loss. Introns occurring at approximately the same location (hereafter called a ``match') were tallied for each of the
paired proteins. Matches were identified by their positions in the multiple alignment and were defined as any two introns
occurring within a window of 11 possible nucleotide positions, thereby allowing for possible alignment errors and ``intron
sliding.' Matches were tallied from the raw data and compared with the expected number of matches for the two different scenarios.
The results suggest that the distribution of introns in genes encoding proteins is due to random insertion and not random
loss.
Received: 8 September 1996 / Accepted: 24 January 1997 相似文献
279.
We have determined the genomic structure of an integrin β-subunit gene from the coral, Acropora millepora. The coding region of the gene contains 26 introns, spaced relatively uniformly, and this is significantly more than have
been found in any integrin β-subunit genes from higher animals. Twenty-five of the 26 coral introns are also found in a β-subunit
gene from at least one other phylum, indicating that the coral introns are ancestral. While there are some suggestions of
intron gain or sliding, the predominant theme seen in the homologues from higher animals is extensive intron loss. The coral
baseline allows one to infer that a number of introns found in only one phylum of higher animals result from frequent intron
loss, as opposed to the seemingly more parsimonious alternative of isolated intron gain. The patterns of intron loss confirm
results from protein sequences that most of the vertebrate genes, with the exception of β4, belong to one of two β subunit
families. The similarity of the patterns within each of the β1,2,7 and β3,5,6,8 groups indicates that these gene structures
have been very stable since early vertebrate evolution. Intron loss has been more extensive in the invertebrate genes, and
obvious patterns have yet to emerge in this more limited data set.
Received: 5 March 2001 / Accepted: 17 May 2001 相似文献
280.
B. Sargueil A. Delahodde D. Hatat G. L. Tian J. Lazowska C. Jacg 《Molecular & general genetics : MGG》1991,225(2):340-341
Summary Two group I intron-encoded proteins from the yeast mitochondrial genome have already been shown to have a specific DNA endonuclease activity. This activity mediates intron insertion by cleaving the DNA sequence corresponding to the splice junction of an intronless strain. We have discovered in mitochondrial extracts from the yeast strain 777-3A a new DNA endonuclease activity which cleaves the fused exon A3-exon A4 junction sequence of the COXI gene. 相似文献