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1.
In translation, separate aminoacyl-tRNA synthetases attach the 20 different amino acids to their cognate tRNAs, with the
exception of glutamine. Eukaryotes and some bacteria employ a specific glutaminyl-tRNA synthetase (GlnRS) which other Bacteria,
the Archaea (archaebacteria), and organelles apparently lack. Instead, tRNAGln is initially acylated with glutamate by glutamyl-tRNA synthetase (GluRS), then the glutamate moiety is transamidated to glutamine.
Lamour et al. [(1994) Proc Natl Acad Sci USA 91:8670–8674] suggested that an early duplication of the GluRS gene in eukaryotes
gave rise to the gene for GlnRS—a copy of which was subsequently transferred to proteobacteria. However, questions remain
about the occurrence of GlnRS genes among the Eucarya (eukaryotes) outside of the ``crown' taxa (animals, fungi, and plants),
the distribution of GlnRS genes in the Bacteria, and their evolutionary relationships to genes from the Archaea. Here, we
show that GlnRS occurs in the most deeply branching eukaryotes and that putative GluRS genes from the Archaea are more closely
related to GlnRS and GluRS genes of the Eucarya than to those of Bacteria. There is still no evidence for the existence of
GlnRS in the Archaea. We propose that the last common ancestor to contemporary cells, or cenancestor, used transamidation
to synthesize Gln-tRNAGln and that both the Bacteria and the Archaea retained this pathway, while eukaryotes developed a specific GlnRS gene through
the duplication of an existing GluRS gene. In the Bacteria, GlnRS genes have been identified in a total of 10 species from
three highly diverse taxonomic groups: Thermus/Deinococcus, Proteobacteria γ/β subdivision, and Bacteroides/Cytophaga/Flexibacter.
Although all bacterial GlnRS form a monophyletic group, the broad phyletic distribution of this tRNA synthetase suggests that
multiple gene transfers from eukaryotes to bacteria occurred shortly after the Archaea–eukaryote divergence. 相似文献
2.
In this work detailed statistics on ancestral gene duplication and gene conservation in completely sequenced cellular genomes
are presented. Analysis of open reading frame (ORF) products having simultaneous matches in several distinct organisms showed
a significant correlation between duplication and conservation. Systematic comparisons of predicted proteomes of 23 organisms
(including 20 that have been completely sequenced), have allowed us to quantify the degree of ancestral duplication within
each genome and the level of conservation between genomes, using threshold values calculated for individual organisms. Statistical
analysis of various gene proportions revealed interesting trends in gene structure and evolution, such as that (a) more than
one-quarter (25%–66%) of the predicted ORF products of the surveyed organisms are not unique, indicating a high level of ancestral
duplications; (b) levels of exclusive conservation within Bacteria are higher than those within the eukaryal or archaeal domains;
and (c) at least one-half (47–99%) of the total predicted ORF products in the surveyed genomes have one or several highly
significant matches in another genome. Significant matches are based on simulations taking into account the mean size of ORF
products and the composition of each target organism's proteome. The methodology we have developed ensures stability and comparability
of our results as the number of completely sequenced genomes increases.
Received: 4 May 1998 / Accepted: 28 September 1998 相似文献
3.
In bacteria, synonymous codon usage can be considerably affected by base composition at neighboring sites. Such context-dependent
biases may be caused by either selection against specific nucleotide motifs or context-dependent mutation biases. Here we
consider the evolutionary conservation of context-dependent codon bias across 11 completely sequenced bacterial genomes. In
particular, we focus on two contextual biases previously identified in Escherichia coli; the avoidance of out-of-frame stop codons and AGG motifs. By identifying homologues of E. coli genes, we also investigate the effect of gene expression level in Haemophilus influenzae and Mycoplasma genitalium. We find that while context-dependent codon biases are widespread in bacteria, few are conserved across all species considered.
Avoidance of out-of-frame stop codons does not apply to all stop codons or amino acids in E. coli, does not hold for different species, does not increase with gene expression level, and is not relaxed in Mycoplasma spp., in which the canonical stop codon, TGA, is recognized as tryptophan. Avoidance of AGG motifs shows some evolutionary
conservation and increases with gene expression level in E. coli, suggestive of the action of selection, but the cause of the bias differs between species. These results demonstrate that
strong context-dependent forces, both selective and mutational, operate on synonymous codon usage but that these differ considerably
between genomes.
Received: 6 May 1999 / Accepted: 29 October 1999 相似文献
4.
Janet L. Siefert Kirt A. Martin Fadi Abdi William R. Widger George E. Fox 《Journal of molecular evolution》1997,45(5):467-472
Five complete bacterial genome sequences have been released to the scientific community. These include four (eu)Bacteria,
Haemophilus influenzae, Mycoplasma genitalium, M. pneumoniae, and Synechocystis PCC 6803, as well as one Archaeon, Methanococcus jannaschii. Features of organization shared by these genomes are likely to have arisen very early in the history of the bacteria and
thus can be expected to provide further insight into the nature of early ancestors. Results of a genome comparison of these
five organisms confirm earlier observations that gene order is remarkably unpreserved. There are, nevertheless, at least 16
clusters of two or more genes whose order remains the same among the four (eu)Bacteria and these are presumed to reflect conserved
elements of coordinated gene expression that require gene proximity. Eight of these gene orders are essentially conserved
in the Archaea as well. Many of these clusters are known to be regulated by RNA-level mechanisms in Escherichia coli, which supports the earlier suggestion that this type of regulation of gene expression may have arisen very early. We conclude
that although the last common ancestor may have had a DNA genome, it likely was preceded by progenotes with an RNA genome.
Received: 10 March 1996 / Accepted: 20 May 1997 相似文献
5.
It has been hypothesized that a large fraction of 24% noncoding DNA in R. prowazekii consists of degraded genes. This hypothesis has been based on the relatively high G+C content of noncoding DNA. However,
a comparison with other genomes also having a low overall G+C content shows that this argument would also apply to other bacteria.
To test this hypothesis, we study the coding potential in sets of genes, pseudogenes, and intergenic regions. We find that
the correlation function and the χ2-measure are clearly indicative of the coding function of genes and pseudogenes. However, both coding potentials make almost
no indication of a preexisting reading frame in the remaining 23% of noncoding DNA. We simulate the degradation of genes due
to single-nucleotide substitutions and insertions/deletions and quantify the number of mutations required to remove indications
of the reading frame. We discuss a reduced selection pressure as another possible origin of this comparatively large fraction
of noncoding sequences.
Received: 27 December 1999 / Accepted: 5 July 2000 相似文献
6.
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
7.
8.
In our previous studies on sheep parotid secretory cells, we showed that the K+ current evoked by acetylcholine (ACh) was not carried by the high-conductance voltage- and Ca2+-activated K+ (BK) channel which is so conspicuous in unstimulated cells, notwithstanding that the BK channel is activated by ACh. Since
several studies from other laboratories had suggested that the BK channel did carry the ACh-evoked K+ current in the secretory cells of the mouse mandibular gland, and that the current could be blocked with tetraethylammonium
(TEA), a known blocker of BK channels, we decided to investigate the ACh-evoked K+ current in mouse cells more closely. We studied whether the ACh-evoked K+ current in the mouse is inhibited by TEA and quinine. Using the whole-cell patch-clamp technique and microspectrofluorimetric
measurement of intracellular Ca2+, we found that TEA and quinine do inhibit the ACh-evoked K+ current but that the effect is due to inhibition of the increase in intracellular Ca2+ evoked by ACh, not to blockade of a K+ conductance. Furthermore, we found that the K+ conductance activated when ionomycin is used to increase intracellular free Ca2+ was inhibited only by quinine and not by TEA. We conclude that the ACh-evoked K+ current in mouse mandibular cells does not have the blocker sensitivity pattern that would be expected if it were being carried
by the high-conductance, voltage- and Ca2+-activated K+ (BK) channel. The properties of this current are, however, consistent with those of a 40 pS K+ channel that we have reported to be activated by ACh in these cells [16].
Received: 9 January 1996/Revised: 17 April 1996 相似文献
9.
If lateral gene transfer (LGT) has affected all genes over the course of prokaryotic evolution, reconstruction of organismal
phylogeny is compromised. However, if a core of genes is immune to transfer, then the evolutionary history of that core might
be our most reliable guide to the evolution of organisms. Such a core should be preferentially included in the subset of genes
shared by all organisms, but where universally conserved genes have been analyzed, there is too little phylogenetic signal
to allow determination of whether or not they indeed have the same history (Hansmann and Martin 2000; Teichmann and Mitchison
1999). Here we look at a more restricted set, 521 homologous genes (COGs) simultaneously present in four sequenced euryarchaeal
genomes. Although there is overall little robust phylogenetic signal in this data set, there is, among well-supported trees,
strong representation of all three possible four-taxon topologies. ``Informational' genes seem no less subject to LGT than
are ``operational genes,' within the euryarchaeotes. We conclude that (i) even in this collection of conserved genes there
has been extensive LGT (orthologous gene replacement) and (ii) the notion that there is a core of nontransferable genes (the
``core hypothesis') has not been proven and may be unprovable.
Received: 7 November 2000 / Accepted: 20 February 2001 相似文献
10.
Computer analyses of various genome sequences revealed the existence of certain periodical patterns of adenine–adenine dinucleotides
(ApA). For each genome sequence of 13 eubacteria, 3 archaebacteria, 10 eukaryotes, 60 mitochondria, and 9 chloroplasts, we
counted frequencies of ApA dinucleotides at each downstream position within 50 bp from every ApA. We found that the complete
genomes of all three archaebacteria have clear ApA periodicities of about 10 bps. On the other hand, all of the 13 eubacteria
we analyzed were found to have an ApA periodicity of about 11 bp. Similar periodicities exist in the 10 eukaryotes, although
higher organisms such as primates tend to have weaker periodic patterns. None of the mitochondria and chroloplasts we analyzed
showed an evident periodic pattern.
Received: 3 November 1998 / Accepted: 24 March 1999 相似文献
11.
12.
Sawada H Kanaya S Tsuda M Suzuki F Azegami K Saitou N 《Journal of molecular evolution》2002,54(4):437-457
Phytopathogenic Pseudomonas syringae is subdivided into about 50 pathovars due to their conspicuous differentiation with regard to pathogenicity. Based on the
results of a phylogenetic analysis of four genes (gyrB, rpoD, hrpL, and hrpS), Sawada et al. (1999) showed that the ancestor of P. syringae had diverged into at least three monophyletic groups during its evolution. Physical maps of the genomes of representative
strains of these three groups were constructed, which revealed that each strain had five rrn operons which existed on one circular genome. The fact that the structure and size of genomes vary greatly depending on the
pathovar shows that P. syringae genomes are quite rich in plasticity and that they have undergone large-scale genomic rearrangements. Analyses of the codon
usage and the GC content at the codon third position, in conjunction with phylogenomic analyses, showed that the gene cluster
involved in phaseolotoxin synthesis (argK–tox cluster) expanded its distribution by conducting horizontal transfer onto the genomes of two P. syringae pathovars (pv. actinidiae and pv. phaseolicola) from bacterial species distantly related to P. syringae and that its acquisition was quite recent (i.e., after the ancestor of P. syringae diverged into the respective pathovars). Furthermore, the results of a detailed analysis of argK [an anabolic ornithine carbamoyltransferase (anabolic OCTase) gene], which is present within the argK–tox cluster, revealed the plausible process of generation of an unusual composition of the OCTase genes on the genomes of these
two phaseolotoxin-producing pathovars: a catabolic OCTase gene (equivalent to the orthologue of arcB of P. aeruginosa) and an anabolic OCTase gene (argF), which must have been formed by gene duplication, have first been present on the genome of the ancestor of P. syringae; the catabolic OCTase gene has been deleted; the ancestor has diverged into the respective pathovars; the foreign-originated
argK–tox cluster has horizontally transferred onto the genomes of pv. actinidiae and pv. phaseolicola; and hence two copies of only the anabolic OCTase genes (argK and argF) came to exist on the genomes of these two pathovars. Thus, the horizontal gene transfer and the genomic rearrangement were
proven to have played an important role in the pathogenic differentiation and diversification of P. syringae.
Received: 22 May 2001 / Accepted: 26 September 2001 相似文献
13.
We previously reported the sequence of a 9260-bp fragment of mitochondrial (mt) DNA of the cephalopod Loligo bleekeri [J. Sasuga et al. (1999) J. Mol. Evol. 48:692–702]. To clarify further the characteristics of Loligo mtDNA, we have sequenced an 8148-bp fragment to reveal the complete mt genome sequence. Loligo mtDNA is 17,211 bp long and possesses a standard set of metazoan mt genes. Its gene arrangement is not identical to any other
metazoan mt gene arrangement reported so far. Three of the 19 noncoding regions longer than 10 bp are 515, 507, and 509 bp
long, and their sequences are nearly identical, suggesting that multiplication of these noncoding regions occurred in an ancestral
Loligo mt genome. Comparison of the gene arrangements of Loligo, Katharina tunicata, and Littorina saxatilis mt genomes revealed that 17 tRNA genes of the Loligo mt genome are adjacent to noncoding regions. A majority (15 tRNA genes) of their counterparts is found in two tRNA gene clusters
of the Katharina mt genome. Therefore, the Loligo mt genome (17 tRNA genes) may have spread over the genome, and this may have been coupled with the multiplication of the
noncoding regions. Maximum likelihood analysis of mt protein genes supports the clade Mollusca + Annelida + Brachiopoda but
fails to infer the relationships among Katharina, Loligo, and three gastropod species.
Received: 9 May 2001 / Accepted: 3 October 2001 相似文献
14.
C. Bookstein M.W. Musch Y. Xie M.C. Rao E.B. Chang 《The Journal of membrane biology》1999,171(1):87-95
Until recently, studies to characterize the intestinal epithelial Na+/H+ exchangers had to be done in nonepithelial, mutated fibroblasts. In these cells, detection of any Na+/H+ exchange activity requires prior acid loading. Furthermore, most of these experiments used intracellular pH changes to measure
NHE activity. Because changes in pH
i
only approximate Na+/H+ exchange activity, and may be confounded by alterations in buffering capacity and/or non-NHE contributions to pH regulation,
we have used 22[Na] unidirectional apical to cell uptake to measure activities specific to NHE2 or NHE3. Furthermore, we performed these
measurements under basal, nonacid-stimulated conditions to avoid bias from this nonphysiological experimental precondition. Both brush border NHEs, when expressed in
the well-differentiated, intestinal villuslike Caco-2 subclone, C2bbe (C2), localize to the C2 apical domain and are regulated
by second messengers in the same way they are regulated in vivo. Increases in intracellular calcium and cAMP inhibit both isoforms, while phorbol ester affects only NHE3. NHE2 inhibition
by cAMP and Ca++ involves changes to both K
Na
and V
max
. In contrast, the same two second messengers inhibit NHE3 by a decrease in V
max
exclusively. Phorbol ester activation of protein kinase C alters both V
max
and K
Na
of NHE3, suggesting a multilevel regulatory mechanism. We conclude that NHE2 and NHE3, in epithelial cells, are basally active
and are differentially regulated by signal transduction pathways.
Received: 28 January 1999/Revised: 18 May 1999 相似文献
15.
New Glycoprotein-Associated Amino Acid Transporters 总被引:2,自引:0,他引:2
Verrey F Jack DL Paulsen IT Saier MH Pfeiffer R 《The Journal of membrane biology》1999,172(3):181-192
The L-type amino acid transporter LAT1 has recently been identified as being a disulfide-linked ``light chain' of the ubiquitously
expressed glycoprotein 4F2hc/CD98. Several LAT1-related transporters have been identified, which share the same putative 12-transmembrane
segment topology and also associate with the single transmembrane domain 4F2hc protein. They display differing amino acid
substrate specificities, transport kinetics and localizations such as, for instance, y+LAT1 which is localized at the basolateral membrane of transporting epithelia, and the defect of which causes lysinuric protein
intolerance. The b0,+AT transporter which associates with the 4F2hc-related rBAT protein to form the luminal high-affinity diamino acid transporter
defective in cystinuria, belongs to the same family of glycoprotein-associated amino acid transporters (gpaATs). These glycoprotein-associated
transporters function as amino acid exchangers. They extend the specificity range of vectorial amino acid transport when located
in the same membrane as carriers that unidirectionally transport one of the exchanged substrates. gpaATs belong to a phylogenetic
cluster within the amino acid/polyamine/choline (APC) superfamily of transporters. This cluster, which we designate the LAT
family (named after its first vertebrate member), includes some members from nematodes, yeast and bacteria. The latter of
these proteins presumably lack association with a second subunit. In this review, we focus on the animal members of the LAT
cluster that form, together with some of the nematode members, the family of glycoprotein-associated amino acid transporters
(gpaAT family).
Received: 20 July 1999/Revised: 7 September 1999 相似文献
16.
Ca2+-regulated motility is essential to numerous cellular functions, including muscle contraction. Systems with troponin C, myosin
light chain, or calmodulin as the Ca2+ receptor have evolved in striated muscle and other types of cells to transduce the cytoplasm Ca2+ signals into allosteric conformational changes of contractile proteins. While these Ca2+ receptors are homologous proteins, their coupling to the responding elements is quite different in various cell types. The
Ca2+ regulatory system in vertebrate striated muscle represents a highly specialized such signal transduction pathway consisting
of the troponin complex and tropomyosin associated with the actin filament. To understand the molecular mechanism in the Ca2+ regulation of muscle contraction and cell motility, we have revealed a preserved ancestral close linkage between the genes
encoding two of the troponin subunits, troponin I and troponin T, in the genome of mouse. The data suggest that the troponin
I and troponin T genes may have originated from a single locus and evolved in parallel to encode a striated muscle-specific
adapter to couple the Ca2+ receptor, troponin C, to the actin–myosin contractile machinery. This hypothesis views the three troponin subunits as two
structure–function domains: the Ca2+ receptor and the signal transducing adapter. This model may help to further our understanding of the Ca2+ regulation of muscle contraction and the structure–function relationship of other potential adapter proteins which are converged
to constitute the Ca2+ signal transduction pathways governing nonmuscle cell motility.
Received: 15 April 1999 / Accepted: 15 July 1999 相似文献
17.
Based on 152 mitochondrial genomes and 36 bacterial chromosomes that have been completely sequenced, as well as three long
contigs for human chromosomes 6, 21, and 22, we examined skews of mononucleotide frequencies and the relative abundance of
dinucleotides in one DNA strand. Each group of these genomes has its own characteristics. Regarding mitochondrial genomes,
both CpG and GpT are underrepresented, while either GpG or CpC or both are overrepresented. The relative frequency of nucleotide T vs A and of nucleotide G vs C is strongly skewed, due
presumably to strand asymmetry in replication errors and unidirectional DNA replication from single origins. Exceptions are
found in the plant and yeast mitochondrial genomes, each of which may replicate from multiple origins. Regarding bacterial
genomes, the ``universal' rule of CpG deficiency is restricted to archaebacteria and some eubacteria. In other eubacteria, the most underrepresented dinucleotide
is either TpA or GpT. In general, there are significant T vs A and G vs C skews in each half of the bacterial genome, although these are almost
exactly canceled out over the whole genome. Regarding human chromosomes 6, 21, and 22, dinucleotide CpG tends to be avoided. The relative frequency of mononucleotides exhibits conspicuous local skews, suggesting that each of
these chromosomal segments contains more than one DNA replication origin. It is concluded that, when there are several replicons
in a genomic region, not only the number of DNA replication origins but also the directionality is important and that the
observed patterns of nucleotide frequencies in the genome strongly support the hypothesis of strand asymmetry in replication
errors.
Received: 1 November 2000 / Accepted: 12 March 2001 相似文献
18.
F.L. González Flecha P.R. Castello J.J. Gagliardino J.P.F.C. Rossi 《The Journal of membrane biology》1999,171(1):25-34
We have previously demonstrated (Diabetes
39:707–711, 1990) that in vitro glycation of the red cell Ca2+ pump diminishes the Ca2+-ATPase activity of the enzyme up to 50%. Such effect is due to the reaction of glucose with lysine residues of the Ca2+ pump (Biochem. J.
293:369–375, 1993). The aim of this work was to determine whether the effect of glucose is due to a full inactivation of a fraction
of the total population of Ca2+ pump, or to a partial inactivation of all the molecules. Glycation decreased the V
max
for the ATPase activity leaving unaffected the apparent affinities for Ca2+, calmodulin or ATP. The apparent turnover was identical in both, the glycated and the native enzyme. Glycation decreased
the V
max
for the ATP-dependent but not for the calmodulin-activated phosphatase activities. Concomitantly with the inhibition, up
to 6.5% of the lysine residues were randomly glycated. The probabilistic analysis of the relation between the enzyme activity
and the fraction of nonmodified residues indicates that only one Lys residue is responsible for the inhibition. We suggest
that glucose decreases the Ca2+-ATPase activity by reacting with one essential Lys residue probably located in the vicinity of the catalytic site, which
results in the full inactivation of the enzyme. Thus, Ca2+-ATPase activity measured in erythrocyte membranes or purified enzyme preparations preincubated with glucose depends on the
remaining enzyme molecules in which the essential Lys residue stays unglycated.
Received: 9 March 1999/Revised: 11 May 1999 相似文献
19.
The rate-limiting step for the maternofetal exchange of low molecular-weight solutes in humans is constituted by transport
across a single epithelial layer (syncytiotrophoblast) of the placenta. Other than the well-established presence of a large-conductance,
multisubstate Cl− channel, the ionic channels occurring in this syncytial tissue are, for the most part, unknown. We have found that fusion
of apical plasma membrane-enriched vesicle fractions with planar lipid bilayers leads, mainly (96% of 353 reconstitutions),
to the reconstitution of nonselective cation channels. Here we describe the properties of this novel placental conductance
at the single-channel level. The channel has a large (>200 pS) and variable conductance, is cation selective (P
Cl
/P
K
≅ 0.024), is reversibly inhibited (presumably blocked) by submillimolar La3+, has very unstable kinetics, and displays a large number (>10) of current sublevels with a ``promiscuous' connectivity pattern.
The occurrence of both ``staircaselike' and ``all-or-nothing' transitions between the minimum and maximum current levels
was intriguing, particularly considering the large number of conductance levels spanned at a time during the concerted current
steps. Single-channel data simulated according to a multistate linear reaction scheme, with rate constants that can vary spontaneously
in time, reproduce many aspects of the recorded subconductance behavior. The channel's sensitivity to lanthanides is reminiscent
of stretch-sensitive channels which, in turn, suggests a physiological role for this ion channel as a mechanotransducer during
syncytiotrophoblast-volume regulation.
Received: 30 August 1999/Revised: 12 November 1999 相似文献
20.
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