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1.
Nuclear and Nucleomorph SSU rDNA Phylogeny in the Cryptophyta and the Evolution of Cryptophyte Diversity 总被引:5,自引:0,他引:5
The plastid-bearing members of the Cryptophyta contain two functional eukaryotic genomes of different phylogenetic origin,
residing in the nucleus and in the nucleomorph, respectively. These widespread and diverse protists thus offer a unique opportunity
to study the coevolution of two different eukaryotic genomes within one group of organisms. In this study, the SSU rRNA genes
of both genomes were PCR-amplified with specific primers and phylogenetic analyses were performed on different data sets using
different evolutionary models. The results show that the composition of the principal clades obtained from the phylogenetic
analyses of both genes was largely congruent, but striking differences in evolutionary rates were observed. These affected
the topologies of the nuclear and nucleomorph phylogenies differently, resulting in long-branch attraction artifacts when
simple evolutionary models were applied. Deletion of long-branch taxa stabilized the internal branching order in both phylogenies
and resulted in a completely resolved topology in the nucleomorph phylogeny. A comparison of the tree topologies derived from
SSU rDNA sequences with characters previously used in cryptophyte systematics revealed that the biliprotein type was congruent,
but the type of inner periplast component incongruent, with the molecular trees. The latter is indicative of a hidden cellular
dimorphism (cells with two periplast types present in a single clonal strain) of presumably widespread occurrence throughout
cryptophyte diversity, which, in consequence, has far-reaching implications for cryptophyte systematics as it is practiced
today. 相似文献
2.
The Nonrandom Location of Synonymous Codons Suggests That Reading Frame-Independent Forces Have Patterned Codon Preferences 总被引:6,自引:0,他引:6
Biased codon usage is common in eukaryotic and prokaryotic genes. Evidence from Escherichia, Saccharomyces, and Drosophila indicates that it favors translational efficiency and accuracy. However, to date no functional advantages have been identified
in the codon–anticodon interactions involving the most frequently used (preferred) codons. Here we present evidence that forces
not related to the individual codon–anticodon interaction may be involved in determining which synonymous codons are preferred
or avoided. We show that the ``off-frame' trinucleotide motif preferences inferrable from Drosophila coding regions are often in the same direction as Drosophila's ``in-frame' codon preferences, i.e., its codon usage. The off-frame preferences were inferred from the nonrandomness of
the location of confamilial synonymous codons along coding regions—a pattern often described as a context dependence of nucleotide
choice at synonymous positions or as codon-pair bias. We relied on randomizations of the location of confamilial codons that
do not alter, and cannot be influenced by, the encoded amino acid sequences, codon usage, or base composition of the genes
examined. The statistically significant congruency of in-frame and off-frame trinucleotide preferences suggests that the same
kind of reading-frame-independent force(s) may also influence synonymous codon choice. These forces may have produced biases
in codon usage that then led to the evolution of the translational advantages of these motifs as preferred codons. Under this
scenario, tRNA pool size differences between preferred and nonpreferred codons initially were evolved to track the default
overrepresentation of codons with preferred motifs. The motif preference hypothesis can explain the structuring of codon preferences
and the similarities in the codon usages of distantly related organisms.
Received: 10 November 1998 / Accepted: 23 February 1999 相似文献
3.
Here we describe a new short retroposon family of rodents. Like the primate Alu element consisting of two similar monomers,
it is dimeric, but the left and right monomers are different and descend from B1 and ID short retroposons, respectively. Such
elements (B1-dID) were found in the genomes of Gliridae, Sciuridae, Castoridae, Caviidae, and Hystricidae. Nucleotide sequences
of this retroposon can be assigned to several structural variants. Phylogenetic analysis of B1-dID and related sequences suggests
a possible scenario of B1-dID evolution in the context of rodent evolution.
Received: 30 August 1999 / Accepted: 20 March 2000 相似文献
4.
Phylogenetic Relationships of Acanthocephala Based on Analysis of 18S Ribosomal RNA Gene Sequences 总被引:3,自引:0,他引:3
García-Varela M Pérez-Ponce de León G de la Torre P Cummings MP Sarma SS Laclette JP 《Journal of molecular evolution》2000,50(6):532-540
Acanthocephala (thorny-headed worms) is a phylum of endoparasites of vertebrates and arthropods, included among the most
phylogenetically basal tripoblastic pseudocoelomates. The phylum is divided into three classes: Archiacanthocephala, Palaeacanthocephala,
and Eoacanthocephala. These classes are distinguished by morphological characters such as location of lacunar canals, persistence
of ligament sacs in females, number and type of cement glands in males, number and size of proboscis hooks, host taxonomy,
and ecology. To understand better the phylogenetic relationships within Acanthocephala, and between Acanthocephala and Rotifera,
we sequenced the nearly complete 18S rRNA genes of nine species from the three classes of Acanthocephala and four species
of Rotifera from the classes Bdelloidea and Monogononta. Phylogenetic relationships were inferred by maximum-likelihood analyses
of these new sequences and others previously determined. The analyses showed that Acanthocephala is the sister group to a
clade including Eoacanthocephala and Palaeacanthocephala. Archiacanthocephala exhibited a slower rate of evolution at the
nucleotide level, as evidenced by shorter branch lengths for the group. We found statistically significant support for the
monophyly of Rotifera, represented in our analysis by species from the clade Eurotatoria, which includes the classes Bdelloidea
and Monogononta. Eurotatoria also appears as the sister group to Acanthocephala.
Received: 12 October 1999 / Accepted: 8 February 2000 相似文献
5.
Molecular Phylogeny of Metazoan Intermediate Filament Proteins 总被引:7,自引:0,他引:7
Andreas Erber Dieter Riemer Marc Bovenschulte Klaus Weber 《Journal of molecular evolution》1998,47(6):751-762
We have cloned cytoplasmic intermediate filament (IF) proteins from a large number of invertebrate phyla using cDNA probes,
the monoclonal antibody IFA, peptide sequence information, and various RT-PCR procedures. Novel IF protein sequences reported
here include the urochordata and nine protostomic phyla, i.e., Annelida, Brachiopoda, Chaetognatha, Echiura, Nematomorpha,
Nemertea, Platyhelminthes, Phoronida, and Sipuncula. Taken together with the wealth of data on IF proteins of vertebrates
and the results on IF proteins of Cephalochordata, Mollusca, Annelida, and Nematoda, two IF prototypes emerge. The L-type,
which includes 35 sequences from 11 protostomic phyla, shares with the nuclear lamins the long version of the coil 1b subdomain
and, in most cases, a homology segment of some 120 residues in the carboxyterminal tail domain. The S-type, which includes
all four subfamilies (types I to IV) of vertebrate IF proteins, lacks 42 residues in the coil 1b subdomain and the carboxyterminal
lamin homology segment. Since IF proteins from all three phyla of the chordates have the 42-residue deletion, this deletion
arose in a progenitor prior to the divergence of the chordates into the urochordate, cephalochordate, and vertebrate lineages,
possibly already at the origin of the deuterostomic branch. Four phyla recently placed into the protostomia on grounds of
their 18S rDNA sequences (Brachiopoda, Nemertea, Phoronida, and Platyhelminthes) show IF proteins of the L-type and fit by
sequence identity criteria into the lophotrochozoic branch of the protostomia.
Received: 2 April 1998 / Accepted: 19 June 1998 相似文献
6.
Varela MF Wilson TH Rodon-Rivera V Shepherd S Dehne TA Rector AC 《The Journal of membrane biology》2000,174(3):199-205
Lactose and melibiose are actively accumulated by the wild-type Escherichia coli lactose carrier, which is an integral membrane protein energized by the proton motive force. Mutants of the E. coli lactose carrier were isolated by their ability to grow on minimal plates with succinate plus IPTG in the presence of the
toxic lactose analog β-thio-o-nitrophenylgalactoside (TONPG). TONPG-resistant mutants were streaked on melibiose MacConkey indicator plates, and red clones
were picked. These melibiose positive mutants were then streaked on lactose MacConkey plates, and white clones were picked.
Transport assays indicated that the mutants had altered sugar recognition and a defect in sugar accumulation. The mutants
had a poor apparent K
m
for both lactose and melibiose in transport. One mutant had almost no ability to take up lactose, but melibiose downhill
transport was 58% (V
max
) of normal. All of the mutants accumulated methyl-α-d-galactopyranoside (TMG) to only 8% or less of normal, and two failed to accumulate. Immunoblot analysis of the mutant lactose
carrier proteins indicated that loss of sugar transport activity was not due to loss of expression in the membrane. Nucleotide
sequencing of the lacY gene from the mutants revealed changes in the following amino acids of the lactose carrier: M23I, W151L, G257D, A295D and
G377V. Two of the mutants (G257D and G377V) are novel in that they represent the first amino acids in periplasmic loops to
be implicated with changes in sugar recognition. We conclude that the amino acids M23, W151, G257, A295 and G377 of the E. coli lactose carrier play either a direct or an indirect role in sugar recognition and accumulation.
Received: 12 October 1999/Revised: 21 December 1999 相似文献
7.
Aphids belonging to the three genera Tuberaphis, Glyphinaphis, and Cerataphis contain extracellular fungal symbionts that resemble endocellular yeast-like symbionts of planthoppers. Whereas the symbiont
of planthoppers has a uricase (urate oxidase; EC 1.7.3.3) and recycles uric acid that the host stores, no uric acid was found
in Tuberaphis styraci, and its fungal symbiont did not exhibit the uricase activity. However, the fungal symbionts of these aphids, including that
of T. styraci, were shown to have putative uricase genes, or pseudogenes, for the uricase. Sequence analysis of these genes revealed that
deleterious mutations occurred independently on each lineage of Glyphinaphis and Tuberaphis, while no such mutation was found in the lineage of Cerataphis. These genes were almost identical to those cloned from the symbionts of planthoppers, though the host aphids and planthoppers
are phylogenetically distant. To estimate the phylogenetic relationship in detail between the fungal symbionts of aphids and
those of planthoppers, a gene tree was constructed based on the sequences of the uricase genes including their flanking regions.
As a result, the symbionts of planthoppers and Tuberaphis aphids formed a sister group against those of Glyphinaphis and Cerataphis aphids with high bootstrap confidence levels, which strongly suggests that symbionts have been horizontally transferred from
the aphids' lineage to the planthoppers'.
Received: 29 March 2000 / Accepted: 31 May 2000 相似文献
8.
Thomas A. Gorr Barbara K. Mable Traute Kleinschmidt 《Journal of molecular evolution》1998,47(4):471-485
Phylogenetic relationships among reptiles were examined using previously published and newly determined hemoglobin sequences.
Trees reconstructed from these sequences using maximum-parsimony, neighbor-joining, and maximum-likelihood algorithms were
compared with a phylogenetic tree of Amniota, which was assembled on the basis of published morphological data. All analyses differentiated α chains into αA and αD types, which are present in all reptiles except crocodiles, where only αA chains are expressed. The occurrence of the αD chain in squamates (lizards and snakes only in this study) appears to be a general characteristic of these species. Lizards
and snakes also express two types of β chains (βI and βII), while only one type of β chain is present in birds and crocodiles.
Reconstructed hemoglobin trees for both α and β sequences did not yield the monophyletic Archosauria (i.e., crocodilians + birds) and Lepidosauria (i.e., Sphenodon+ squamates) groups defined by the morphology tree. This discrepancy, as well as some other poorly resolved nodes, might be
due to substantial heterogeneity in evolutionary rates among single hemoglobin lineages. Estimation of branch lengths based
on uncorrected amino acid substitutions and on distances corrected for multiple substitutions (PAM distances) revealed that
relative rates for squamate αA and αD chains and crocodilian β chains are at least twice as high as those of the rest of the chains considered. In contrast to
these rate inequalities between reptilian orders, little variation was found within squamates, which allowed determination
of absolute evolutionary rates for this subset of hemoglobins. Rate estimates for hemoglobins of lizards and snakes yielded
1.7 (αA) and 3.3 (β) million years/PAM when calibrated with published divergence time vs. PAM distance correlates for several speciation
events within snakes and for the squamate ↔ sphenodontid split. This suggests that hemoglobin chains of squamate reptiles
evolved ∼3.5 (αA) or ∼1.7 times (β) faster than their mammalian equivalents. These data also were used to obtain a first estimate of some
intrasquamate divergence times.
Received: 15 September 1997 / Accepted: 4 February 1998 相似文献
9.
Langenkämper G Fung RW Newcomb RD Atkinson RG Gardner RC MacRae EA 《Journal of molecular evolution》2002,54(3):322-332
We present phylogenetic analyses to demonstrate that there are three families of sucrose phosphate synthase (SPS) genes present
in higher plants. Two data sets were examined, one consisting of full-length proteins and a second larger set that covered
a highly conserved region including the 14-3-3 binding region and the UDPGlu active site. Analysis of both datasets showed
a well supported separation of known genes into three families, designated A, B, and C. The genomic sequences of Arabidopsis thaliana include a member in each family: two genes on chromosome 5 belong to Family A, one gene on chromosome 1 to Family B, and
one gene on chromosome 4 to Family C. Each of three Citrus genes belong to one of the three families. Intron/exon organization of the four Arabidopsis genes differed according to phylogenetic analysis, with members of the same family from different species having similar
genomic organization of their SPS genes. The two Family A genes on Arabidopsis chromosome 5 appear to be due to a recent duplication. Analysis of published literature and ESTs indicated that functional
differentiation of the families was not obvious, although B family members appear not to be expressed in roots. B family genes
were cloned from two Actinidia species and southern analysis indicated the presence of a single gene family, which contrasts to the multiple members of
Family A in Actinidia. Only two family C genes have been reported to date.
Received: 17 April 2001 / Accepted: 27 August 2001 相似文献
10.
Karen Miller Clare Lynch Joanne Martin Elisabeth Herniou Michael Tristem 《Journal of molecular evolution》1999,49(3):358-366
Gypsy LTR-retrotransposons have been identified in the genomes of many organisms, but only a small number of vertebrate examples
have been reported to date. Here we show that members of this family are likely to be widespread in many vertebrate classes
with the possible exceptions of mammals and birds. Phylogenetic analyses demonstrate that although there are several distinct
lineages of vertebrate gypsy LTR-retrotransposons, the majority clusters into one monophyletic clade. Groups of fungal, plant, and insect elements were
also observed, suggesting horizontal transfer between phyla may be infrequent. However, in contrast to this, there was little
evidence to support sister relationships between elements derived from vertebrate and insect hosts. In fact, the majority
of the vertebrate elements appeared to be most closely related to a group of gypsy LTR-retrotransposons present within fungi. This implies either that at least one horizontal transmission between these two
phyla has occurred previously or that a gypsy LTR-retrotransposon lineage has been lost from insect taxa.
Received: 22 December 1998 / Accepted: 6 April 1999 相似文献
11.
The members of the PKA regulatory subunit family (PKA-R family) were analyzed by multiple sequence alignment and clustering
based on phylogenetic tree construction. According to the phylogenetic trees generated from multiple sequence alignment of
the complete sequences, the PKA-R family was divided into four subfamilies (types I to IV). Members of each subfamily were
exclusively from animals (types I and II), fungi (type III), and alveolates (type IV). Application of the same methodology
to the cAMP-binding domains, and subsequently to the region delimited by β-strands 6 and 7 of the crystal structures of bovine
RIα and rat RIIβ (the phosphate-binding cassette; PBC), proved that this highly conserved region was enough to classify unequivocally
the members of the PKA-R family. A single signature sequence, F–G–E–[LIV]–A–L–[LIMV]–x(3)–[PV]–R–[ANQV]–A, corresponding to
the PBC was identified which is characteristic of the PKA-R family and is sufficient to distinguish it from other members
of the cyclic nucleotide-binding protein superfamily. Specific determinants for the A and B domains of each R-subunit type
were also identified. Conserved residues defining the signature motif are important for interaction with cAMP or for positioning
the residues that directly interact with cAMP. Conversely, residues that define subfamilies or domain types are not conserved
and are mostly located on the loop that connects α-helix B′ and β strand 7.
Received: 2 November 2000/Accepted: 14 June 2001 相似文献
12.
Cell Surface Area Regulation and Membrane Tension 总被引:17,自引:0,他引:17
The beautifully orchestrated regulation of cell shape and volume are central themes in cell biology and physiology. Though
it is less well recognized, cell surface area regulation also constitutes a distinct task for cells. Maintaining an appropriate
surface area is no automatic side effect of volume regulation or shape change. The issue of surface area regulation (SAR)
would be moot if all cells resembled mammalian erythrocytes in being constrained to change shape and volume using existing
surface membrane. But these enucleate cells are anomalies, possessing no endomembrane. Most cells use endomembrane to continually
rework their plasma membrane, even while maintaining a given size or shape. This membrane traffic is intensively studied,
generally with the emphasis on targeting and turnover of proteins and delivery of vesicle contents. But surface area (SA)
homeostasis, including the controlled increase or decrease of SA, is another of the outcomes of trafficking.
Our principal aims, then, are to highlight SAR as a discrete cellular task and to survey evidence for the idea that membrane
tension is central to the task. Cells cannot directly ``measure' their volume or SA, yet must regulate both. We posit that
a homeostatic relationship exists between plasma membrane tension and plasma membrane area, which implies that cells detect
and respond to deviations around a membrane tension set point. Maintenance of membrane strength during membrane turnover,
a seldom-addressed aspect of SA dynamics, we examine in the context of SAR.
SAR occurs in both animal and plant cells. The review shows the latter to be a continuing source of groundbreaking work on
tension-sensitive SAR, but is principally slanted to animal cells.
Received: 1 May 2000/Revised: 14 August 2000 相似文献
13.
The definition of an Ecdysozoa clade among the protostomians, including all phyla with a regularly molted α-chitin-rich cuticle,
has been one of the most provocative hypotheses to arise from recent investigations on animal phylogeny. Here we present evidence
in favor of an arthropod-nematode clade, from the comparison of β-thymosin homologues among the Metazoa. Arthropods and nematodes
share the absence of the highly conserved β-thymosin form found in all other documented bilaterian phyla as well as sponges,
and the possession of a very unusual, internally triplicated homologue of the β-thymosin protein, unknown in other phyla.
We argue that such discrete molecular character is phylogenetically very powerful and provides strong evidence for the monophyly
of an arthropod-nematode clade.
Received: 17 December 1999 / Accepted: 7 July 2000 相似文献
14.
Complete chloroplast 23S rRNA and psbA genes from five peridinin-containing dinoflagellates (Heterocapsa pygmaea, Heterocapsa niei, Heterocapsa rotun-data, Amphidinium carterae, and Protoceratium reticulatum) were amplified by PCR and sequenced; partial sequences were obtained from Thoracosphaera heimii and Scrippsiella trochoidea. Comparison with chloroplast 23S rRNA and psbA genes of other organisms shows that dinoflagellate chloroplast genes are the most divergent and rapidly evolving of all.
Quartet puzzling, maximum likelihood, maximum parsimony, neighbor joining, and LogDet trees were constructed. Intersite rate
variation and invariant sites were allowed for with quartet puzzling and neighbor joining. All psbA and 23S rRNA trees showed peridinin-containing dinoflagellate chloroplasts as monophyletic. In psbA trees they are related to those of chromists and red algae. In 23S rRNA trees, dinoflagellates are always the sisters of
Sporozoa (apicomplexans); maximum likelihood analysis of Heterocapsa triquetra 16S rRNA also groups the dinoflagellate and sporozoan sequences, but the other methods were inconsistent. Thus, dinoflagellate
chloroplasts may actually be related to sporozoan plastids, but the possibility of reproducible long-branch artifacts cannot
be strongly ruled out. The results for all three genes fit the idea that dinoflagellate chloroplasts originated from red algae
by a secondary endosymbiosis, possibly the same one as for chromists and Sporozoa. The marked disagreement between 16S rRNA
trees using different phylogenetic algorithms indicates that this is a rather poor molecule for elucidating overall chloroplast
phylogeny. We discuss possible reasons why both plastid and mitochondrial genomes of alveolates (Dinozoa, Sporozoa and Ciliophora)
have ultra-rapid substitution rates and a proneness to unique genomic rearrangements.
Received: 27 December 1999 / Accepted: 24 March 2000 相似文献
15.
We examined the evolution of the repeat regions of three noncoding microsatellite loci in 58 species of the Polistinae, a
subfamily of wasps that diverged over 140 million years ago. A phylogenetic approach allows two new kinds of approaches to
studying microsatellite evolution: character mapping and comparative analysis. The basic repeat structure of the loci was
highly conserved, but was often punctuated with imperfections that appear to be phylogenetically informative. Repeat numbers
evolved more rapidly than other changes in the repeat region. Changes in number of repeats among species seem consistent with
the stepwise mutation model, which is based on slippage during replication as the main source of mutations. Changes in repeat
numbers can occur even when there are very few tandem repeats but longer repeats, especially perfect repeats led to greater
rates of evolutionary change. Species phylogenetically closer to the one from which we identified the loci had longer stretches
of uninterrupted repeats and more different motifs, but not longer total repeat regions. The number of perfect repeats increased
more often than it decreased. However, there was no evidence that some species have consistently greater numbers of repeats
across loci than other species have, once ascertainment bias is eliminated. We also found no evidence for a population size
effect posited by one form of the directionality hypothesis. Overall, phylogenetic variation in repeat regions can be explained
by adding neutral evolution to what is already known about the mutation process. The life cycle of microsatellites appears
to reflect a balance between growth by slippage and degradation by an essentially irreversible accumulation of imperfections.
Received: 13 April 1999 / Accepted: 8 September 1999 相似文献
16.
The evolutionary relationship of muscle and nonmuscle actin isoforms in deuterostomia was studied by the isolation and characterization
of two actin genes from the cephalochordate Branchiostoma lanceolatum and two from the hemichordate Saccoglossus kowalevskii The Branchiostoma genes specify a muscle and a nonmuscle actin type, respectively. Together with earlier results on muscle actins from vertebrates
and urochordates, a N-terminal sequence signature is defined for chordate muscle actins. These diagnostic amino acid residues
separate the chordates from the echinoderms and other metazoa. Although the two Saccoglossus actins characterized so far lack the diagnostic residues, in line with the presumptive phylogenetic position of hemichordates
outside the chordates, a definitive conclusion can only be expected once the full complement of actin genes of Saccoglossus is established. Comparison of the intron patterns of the various deuterostomic actin genes shows that intron 330-3, which
is present in all vertebrate genes, is conspicuously absent from nonvertebrate genes. The possible origin of this intron is
discussed.
Received: 4 July 1997 / Accepted: 29 August 1997 相似文献
17.
Hiroshi Wada Mari Kobayashi Riki Sato Nori Satoh Hitoshi Miyasaka Yoshihisa Shirayama 《Journal of molecular evolution》2002,54(1):118-128
To test the validity of intron–exon structure as a phylogenetic marker, the intron–exon structure of EF-1α genes was investigated
for starfish, acornworms, ascidians, larvaceans, and amphioxus and compared with that of vertebrates. Of the 11 distinct intron
insertion sites found within the coding regions of the deuterostome EF-1α genes, 7 are shared by several taxa, while the remainder
are unique to certain taxa. Examination of the shared introns of the deuterostome EF-1α gene revealed that independent intron
loss or intron insertion must have occurred in separate lineages of the deuterostome taxa. Maximum parsimony analysis of the
intron–exon data matrix recovered five parsimonious trees (consistency index = 0.867). From this result, we concluded that
the intron–exon structure of deuterostome EF-1α has evolved more dynamically than previously thought, rendering it unsuitable
as a phylogenetic marker. We also reconstructed an evolutionary history of intron insertion–deletion events on the deuterostome
phylogeny, based on several molecular phylogenetic studies. These analyses revealed that the deuterostome EF-1α gene has lost
individual introns more frequently than all introns simultaneously. 相似文献
18.
Calcium-induced fusion of liposomes was studied with a view to understand the role of membrane tension in this process. Lipid
mixing due to fusion was monitored by following fluorescence of rhodamine-phosphatidyl-ethanolamine incorporated into liposomal
membrane at a self-quenching concentration. The extent of lipid mixing was found to depend on the rate of calcium addition:
at slow rates it was significantly lower than when calcium was injected instantly. The vesicle inner volume was then made
accessible to external calcium by adding calcium ionophore A23187. No effect on fusion was observed at high rates of calcium
addition while at slow rates lipid mixing was eliminated. Fusion of labeled vesicles with a planar phospholipid membrane (BLM)
was studied using fluorescence microscopy. Above a threshold concentration specific for each ion, Ca2+, Mg2+, Cd2+ and La3+ induce fusion of both charged and neutral membranes. The threshold calcium concentration required for fusion was found to
be dependent on the vesicle charge, but not on the BLM charge. Pretreatment of vesicles with ionophore and calcium inhibited
vesicle fusion with BLM. This effect was reversible: chelation of calcium prior to the application of vesicle to BLM completely
restored their ability to fuse. These results support the hypothesis that tension in the outer monolayer of lipid vesicle
is a primary reason for membrane destabilization promoting membrane fusion. How this may be a common mechanism for both purely
lipidic and protein-mediated membrane fusion is discussed.
Received: 27 September 1999/Revised: 22 March 2000 相似文献
19.
20.
How did the ``universal' genetic code arise? Several hypotheses have been put forward, and the code has been analyzed extensively
by authors looking for clues to selection pressures that might have acted during its evolution. But this approach has been
ineffective. Although an impressive number of properties has been attributed to the universal code, it has been impossible
to determine whether selection on any of these properties was important in the code's evolution or whether the observed properties
arose as a consequence of selection on some other characteristic. Therefore we turned the question around and asked, what
would a genetic code look like if it had evolved in response to various different selection pressures? To address this question,
we constructed a genetic algorithm. We found first that selecting on a particular measure yields codes that are similar to
each other. Second, we found that the universal code is far from minimized with respect to the effects of mutations (or translation
errors) on the amino acid compositions of proteins. Finally, we found that the codes that most closely resembled real codes
were those generated by selecting on aspects of the code's structure, not those generated by selecting to minimize the effects
of amino acid substitutions on proteins. This suggests that the universal genetic code has been selected for a particular
structure—a structure that confers an important flexibility on the evolution of genes and proteins—and that the particular
assignments of amino acids to codons are secondary.
Received: 29 December 1998 / Accepted: 8 July 1999 相似文献