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An 82-kb Aspergillus parasiticus genomic DNA region representing the completed sequence of the well-organized aflatoxin pathway gene cluster has been sequenced and annotated. In addition to the 19 reported and characterized aflatoxin pathway genes and the four sugar utilization genes in this cluster, we report here the identification of six newly identified genes which are putatively involved in aflatoxin formation. The function of these genes, the cluster organization and its significance in gene expression are discussed. 相似文献
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Aflatoxins produced by Aspergillus flavus are potent carcinogens that contaminate agricultural crops. Recent efforts to reduce aflatoxin concentrations in crops have focused on biological control using nonaflatoxigenic A. flavus strains AF36 (=NRRL 18543) and NRRL 21882 (the active component of afla‐guard®). However, the evolutionary potential of these strains to remain nonaflatoxigenic in nature is unknown. To elucidate the underlying population processes that influence aflatoxigenicity, we examined patterns of linkage disequilibrium (LD) spanning 21 regions in the aflatoxin gene cluster of A. flavus. We show that recombination events are unevenly distributed across the cluster in A. flavus. Six distinct LD blocks separate late pathway genes aflE, aflM, aflN, aflG, aflL, aflI and aflO, and there is no discernable evidence of recombination among early pathway genes aflA, aflB, aflC, aflD, aflR and aflS. The discordance in phylogenies inferred for the aflW/aflX intergenic region and two noncluster regions, tryptophan synthase and acetamidase, is indicative of trans‐species evolution in the cluster. Additionally, polymorphisms in aflW/aflX divide A. flavus strains into two distinct clades, each harbouring only one of the two approved biocontrol strains. The clade with AF36 includes both aflatoxigenic and nonaflatoxigenic strains, whereas the clade with NRRL 21882 comprises only nonaflatoxigenic strains and includes all strains of A. flavus missing the entire gene cluster or with partial gene clusters. Our detection of LD blocks in partial clusters indicates that recombination may have played an important role in cluster disassembly, and multilocus coalescent analyses of cluster and noncluster regions indicate lineage‐specific gene loss in A. flavus. These results have important implications in assessing the stability of biocontrol strains in nature. 相似文献
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Aspergillus flavus isolates produce only aflatoxins B1 and B2, while Aspergillus parasiticus and Aspergillus nomius produce aflatoxins B1, B2, G1, and G2. Sequence comparison of the aflatoxin biosynthesis pathway gene cluster upstream from the polyketide synthase gene, pksA, revealed that A. flavus isolates are missing portions of genes (cypA and norB) predicted to encode, respectively, a cytochrome P450 monooxygenase and an aryl alcohol dehydrogenase. Insertional disruption of cypA in A. parasiticus yielded transformants that lack the ability to produce G aflatoxins but not B aflatoxins. The enzyme encoded by cypA has highest amino acid identity to Gibberella zeae Tri4 (38%), a P450 monooxygenase previously shown to be involved in trichodiene epoxidation. The substrate for CypA may be an intermediate formed by oxidative cleavage of the A ring of O-methylsterigmatocystin by OrdA, the P450 monooxygenase required for formation of aflatoxins B1 and B2. 相似文献
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Ignazio Carbone Jorge H Ramirez-Prado Judy L Jakobek Bruce W Horn 《BMC evolutionary biology》2007,7(1):111
Background
The biosynthesis of aflatoxin (AF) involves over 20 enzymatic reactions in a complex polyketide pathway that converts acetate and malonate to the intermediates sterigmatocystin (ST) and O -methylsterigmatocystin (OMST), the respective penultimate and ultimate precursors of AF. Although these precursors are chemically and structurally very similar, their accumulation differs at the species level for Aspergilli. Notable examples are A. nidulans that synthesizes only ST, A. flavus that makes predominantly AF, and A. parasiticus that generally produces either AF or OMST. Whether these differences are important in the evolutionary/ecological processes of species adaptation and diversification is unknown. Equally unknown are the specific genomic mechanisms responsible for ordering and clustering of genes in the AF pathway of Aspergillus. 相似文献6.
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Aflatoxins are toxic and carcinogenic polyketides produced by several Aspergillus species that are known to contaminate agricultural commodities, posing a serious threat to animal and human health. Aflatoxin (AF) biosynthesis is almost fully characterized and involves the coordinated expression of approximately 25 genes clustered in a 70-kb DNA region. Aspergillus parasiticus is an economically important and common agent of AF contamination. Naturally occurring nonaflatoxigenic strains of A. parasiticus are rarely found and generally produce O-methylsterigmatocystin (OMST), the immediate precursor of AF. To elucidate the evolutionary forces acting to retain AF and OMST pathway extrolites (chemotypes), we sequenced 21 intergenic regions spanning the entire cluster in 24 A. parasiticus isolates chosen to represent the genetic diversity within a single Georgia field population. Linkage disequilibrium analyses revealed five distinct recombination blocks in the A. parasiticus cluster. Phylogenetic network analyses showed a history of recombination between chemotype-specific haplotypes, as well as evidence of contemporary recombination. We performed coalescent simulations of variation in recombination blocks and found an approximately twofold deeper coalescence for cluster genealogies compared to noncluster genealogies, our internal standard of neutral evolution. Significantly deeper cluster genealogies are indicative of balancing selection in the AF cluster of A. parasiticus and are further corroborated by the existence of trans-species polymorphisms and common haplotypes in the cluster for several closely related species. Estimates of Ka/Ks for representative cluster genes provide evidence of selection for OMST and AF chemotypes, and indicate a possible role of chemotypes in ecological adaptation and speciation. 相似文献
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Evolution of the leucine gene cluster in Buchnera aphidicola: insights from chromosomal versions of the cluster
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In Buchnera aphidicola strains associated with the aphid subfamilies Thelaxinae, Lachninae, Pterocommatinae, and Aphidinae, the four leucine genes (leuA, -B, -C, and -D) are located on a plasmid. However, these genes are located on the main chromosome in B. aphidicola strains associated with the subfamilies Pemphiginae and Chaitophorinae. The sequence of the chromosomal fragment containing the leucine cluster and flanking genes has different positions in the chromosome in B. aphidicola strains associated with three tribes of the subfamily Pemphiginae and one tribe of the subfamily Chaitophorinae. Due to the extreme gene order conservation of the B. aphidicola genomes, the variability in the position of the leucine cluster in the chromosome may be interpreted as resulting from independent insertions from an ancestral plasmid-borne leucine gene. These findings do not support a chromosomal origin for the leucine genes in the ancestral B. aphidicola and do support a back transfer evolutionary scenario from a plasmid to the main chromosome. 相似文献
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Roderick Nigel Finn Jelena Kolarevic Heidi Kongshaug Frank Nilsen 《BMC evolutionary biology》2009,9(1):2-12
Background
The multiplicity or loss of the vitellogenin (vtg) gene family in vertebrates has been argued to have broad implications for the mode of reproduction (placental or non-placental), cleavage pattern (meroblastic or holoblastic) and character of the egg (pelagic or benthic). Earlier proposals for the existence of three forms of vertebrate vtgs present conflicting models for their origin and subsequent duplication. 相似文献13.
Method for monitoring deletions in the aflatoxin biosynthesis gene cluster of Aspergillus flavus with multiplex PCR
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The report presents a rapid, inexpensive and simple method for monitoring indels with influence on aflatoxin biosynthesis within Aspergillus flavus populations. PCR primers were developed for 32 markers spaced approximately every 5 kb from 20 kb proximal to the aflatoxin biosynthesis gene cluster to the telomere repeat. This region includes gene clusters required for biosynthesis of aflatoxins and cyclopiazonic acid; the resulting data were named cluster amplification patterns (CAPs). CAP markers are amplified in four multiplex PCRs, greatly reducing the cost and time to monitor indels within this region across populations. The method also provides a practical tool for characterizing intraspecific variability in A. flavus not captured with other methods.
Significance and Impact of the Study
Aflatoxins, potent naturally‐occurring carcinogens, cause significant agricultural problems. The most effective method for preventing contamination of crops with aflatoxins is through use of atoxigenic strains of Aspergillus flavus to alter the population structure of this species and reduce incidences of aflatoxin producers. Cluster amplification pattern (CAP) is a rapid multiplex PCR method for identifying and monitoring indels associated with atoxigenicity in A. flavus. Compared to previous techniques, the reported method allows for increased resolution, reduced cost, and greater speed in monitoring the stability of atoxigenic strains, incidences of indel mediated atoxigenicity and the structure of A. flavus populations. 相似文献14.
Evolution of a D. melanogaster glutamate tRNA gene cluster 总被引:18,自引:0,他引:18
We have determined the DNA sequence of a cloned cluster of essentially identical glutamate tRNA genes of D. melanogaster. The cluster consists of five genes: a gene triplet spanning approximately 0.55 kb followed by a 0.45 kb gene doublet 3.0 kb downstream. The genes are all arranged with the same polarity, do not encode the tRNA CCA end and contain no intervening sequences. Examination of the 5' and 3' sequences immediately flanking each gene reveals a striking pattern of sequence homologies between certain of the genes, which suggests a possible evolutionary history of this gene cluster. We propose that two ancestral genes each gave rise to gene doublets by duplication, while one of these gene pairs then gave rise, in turn, to a trio of genes as a result of unequal crossover. 相似文献
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Aspergillus flavus populations are genetically diverse. Isolates that produce either, neither, or both aflatoxins and cyclopiazonic acid (CPA) are present in the field. We investigated defects in the aflatoxin gene cluster in 38 nonaflatoxigenic A. flavus isolates collected from southern United States. PCR assays using aflatoxin-gene-specific primers grouped these isolates into eight (A-H) deletion patterns. Patterns C, E, G, and H, which contain 40 kb deletions, were examined for their sequence breakpoints. Pattern C has one breakpoint in the cypA 3' untranslated region (UTR) and another in the verA coding region. Pattern E has a breakpoint in the amdA coding region and another in the ver1 5'UTR. Pattern G contains a deletion identical to the one found in pattern C and has another deletion that extends from the cypA coding region to one end of the chromosome as suggested by the presence of telomeric sequence repeats, CCCTAATGTTGA. Pattern H has a deletion of the entire aflatoxin gene cluster from the hexA coding region in the sugar utilization gene cluster to the telomeric region. Thus, deletions in the aflatoxin gene cluster among A. flavus isolates are not rare, and the patterns appear to be diverse. Genetic drift may be a driving force that is responsible for the loss of the entire aflatoxin gene cluster in nonaflatoxigenic A. flavus isolates when aflatoxins have lost their adaptive value in nature. 相似文献
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The human cytochrome P450 2D6 (CYP2D6) is a primary enzyme involved in the metabolism of about 25% of commonly used therapeutic drugs. CYP2D6 belongs to the CYP2D subfamily, a gene cluster located on chromosome 22, which comprises the CYP2D6 gene and pseudogenes CYP2D7P and CYP2D8P. Although the chemical and physiological properties of CYP2D6 have been extensively studied, there has been no study to date on molecular evolution of the CYP2D subfamily in the human genome. Such knowledge could greatly contribute to the understanding of drug metabolism in humans because it makes us to know when and how the current metabolic system has been constructed. The knowledge moreover can be useful to find differences in exogenous substrates in a particular metabolism between human and other animals such as experimental animals. Here, we conducted a preliminary study to investigate the evolution and gene organization of the CYP2D subfamily, focused on humans and four non-human primates (chimpanzees, orangutans, rhesus monkeys, and common marmosets). Our results indicate that CYP2D7P has been duplicated from CYP2D6 before the divergence between humans and great apes, whereas CYP2D6 and CYP2D8P have been already present in the stem lineages of New World monkeys and Catarrhini. Furthermore, the origin of the CYP2D subfamily in the human genome can be traced back to before the divergence between amniotes and amphibians. Our analyses also show that reported chimeric sequences of the CYP2D6 and CYP2D7 genes in the chimpanzee genome appear to be exchanged in its genome database. 相似文献
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Ehrlich KC Montalbano B Boué SM Bhatnagar D 《Applied and environmental microbiology》2005,71(12):8963-8965
Disruption of the aflatoxin biosynthesis cluster gene aflY (hypA) gave Aspergillus parasiticus transformants that accumulated versicolorin A. This gene is predicted to encode the Baeyer-Villiger oxidase necessary for formation of the xanthone ring of the aflatoxin precursor demethylsterigmatocystin. 相似文献