Cloning and analysis of the esterase genes conferring insecticide resistance in the peach-potato aphid, Myzus persicae (Sulzer)

Structure and organization of amplicons containing the E4 esterase genes responsible for insecticide resistance in the aphid Myzus persicae (Sulzer)

Insecticide resistance in the aphid Myzus persicae results primarily from the amplification of genes encoding the insecticide-detoxifying esterase, E4. Here we report the analysis of flanking DNA co-amplified with the E4 gene. The 5' end of this gene has an untranslated leader sequence interspersed by two introns, and the promoter region lacks TATA and CAAT boxes. The DNA breakpoint involved in the generation of the amplification is just upstream (approx. 250 bp) of the putative E4 transcription start site; thus the E4 gene is very close to the 5' end of the approx. 24 kb amplicon. PCR primers specific to the 'novel joint' generated during the amplification have been used to show that a wide range of aphid clones have the same amplicons, arranged as a series of head-to-tail direct repeats. Long-distance mapping has revealed the structure of these repeats. This has important implications for understanding both the generation of the amplified genes and the origin and spread of insecticide resistance in M. persicae.

A novel fusidic acid resistance gene from Streptomyces lividans 66 encodes a highly specific esterase

Resistance to fusidic acid in Streptomyces lividans is due to secretion of an extracellular enzyme (FusH) that converts the steroid antibiotic into an inactive derivative. NH2-terminal and several internal amino acid sequences were prepared from the purified enzyme. Using one of the deduced oligonucleotides to probe a subgenomic DNA library, the fusH gene was cloned and sequenced. Sequence analysis located an ORF which, owing to the presence of two putative start codons, indicates a predicted protein with 520 or 509 amino acids. A signal peptide was identified by aligning the deduced amino acids with the N-terminal sequence determined for the mature enzyme. The C-terminal part of the deduced FusH contains a region of three tandemly repeated stretches of 50 amino acids, which is preceded and followed by amino acids showing high homology with the repeats. FusH was found to share a GDS motif with some deduced esterases. S. lividans transformants carrying fusH on a multicopy vector synthesized high levels of FusH. Purified FusH cleaved equally well an acetyl, a thioacetyl or a formyl group from the 16 beta-position of fusidic acid and its derivatives. However, a propionyl group at the 16 beta-position was attacked with difficulty and a 16 alpha-acetyl group was not hydrolysed at all. These data indicate that FusH is a highly specific esterase. The fusH gene is widely distributed among streptomycetes that modify fusidic acid to its inactive lactone derivative.

cDNA cloning, baculovirus-expression and kinetic properties of the esterase, E3, involved in organophosphorus resistance in Lucilia cuprina

Resistance to organophosphorus insecticides (OPs) in the sheep blowfly, Lucilia cuprina, is associated with a non-staining phenotype of the carboxylesterase isozyme, E3 (E.C. 3.1.1.1). Here, we show that a member of alpha-esterase multigene family, Lc alpha E7, encodes E3. An Lc alpha E7 cDNA has been isolated from an OP-susceptible strain and expressed in a baculovirus. The expressed product is the same as E3 in its electrophoretic mobility and preference for alpha-over beta-naphthyl acetate as substrate. Its preference (kcat/K(m)) for a range of carboxylester substrates is alpha-naphthyl butyrate > alpha-naphthyl propionate > alpha-naphthyl acetate > methylthiobutyrate > p-nitrophenyl acetate. The enzyme is potently inhibited by OPs (ki [paraoxon] = 6.3 +/- 1.4 x 10(7)/M/min, ki [chlorfenvinphos] = 5.9 +/- 0.6 x 10(7)/M/min) and exhibits a high turnover of methylthiobutyrate (1009/s), consistent with its proposed homology to the ali-esterase that is thought to mutate to confer OP resistance in Musca domestica. E3 shares 64% amino acid identity with its Drosophila melanogaster homologue, Dm alpha E7, and is also closely related to other esterases involved in OP resistance such as the B1 esterase of Culex pipiens (38%) and E4 of Myzus persicae (30%).

Duplication and divergence of the genes of the alpha-esterase cluster of Drosophila melanogaster

The alpha-esterase cluster of D. melanogaster contains 11 esterase genes dispersed over 60 kb. Embedded in the cluster are two unrelated open reading frames that have sequence similarity with genes encoding ubiquitin-conjugating enzyme and tropomyosin. The esterase amino acid sequences show 37-66% identity with one another and all but one have all the motifs characteristic of functional members of the carboxyl/cholinesterase multigene family. The exception has several frameshift mutations and appears to be a pseudogene. Patterns of amino acid differences among cluster members in relation to generic models of carboxyl/cholinesterase protein structure are broadly similar to those among other carboxyl/cholinesterases sequenced to date. However the alpha-esterases differ from most other members of the family in: their lack of a signal peptide; the lack of conservation in cysteines involved in disulfide bridges; and in four indels, two of which occur in or adjacent to regions that align with proposed substrate-binding sites of other carboxyl/cholinesterases. Phylogenetic analyses clearly identify three simple gene duplication events within the cluster. The most recent event involved the pseudogene which is located in an intron of another esterase gene. However, relative rate tests suggest that the pseudogene remained functional after the duplication event and has become inactive relatively recently. The distribution of indels also suggests a deeper node in the gene phylogeny that separates six genes at the two ends of the cluster from a block of five in the middle.

Polymerase chain reaction-based identification of insecticide resistance genes and DNA methylation in the aphid Myzus persicae (Sulzer)

The ability of a peach-potato aphid (Myzus persicae) to resist insecticides can depend on both the presence of amplified esterase genes and on differences in their expression. Here we report a polymerase chain reaction-restriction enzyme (PCR-REN) technique which can detect the presence of amplified esterase genes and distinguish between the two possible types of amplified gene (E4 and FE4) and a homologous fragment from susceptible aphids. The technique is quick and sensitive enough to be used on a fraction of an aphid or an individual embryo. Furthermore, it can be combined with a pre-PCR digestion using a methylation-sensitive enzyme (Hpall) to determine whether or not the esterase genes contain 5-methylcytosine, the presence or absence of which is known to correlate with changes in gene expression.

Analysis of amplicons containing the esterase genes responsible for insecticide resistance in the peach-potato aphid Myzus persicae (Sulzer)

The amplification of genes encoding an insecticide-detoxifying esterase (E4) in the peach-potato aphid Myzus persicae is one of the few examples where this genetic phenomenon has been shown to be involved in the response of an intact higher organism to artificial selection. Here we report quantitative and qualitative studies of the repeat units (amplicons) containing the E4 genes in a highly resistant aphid clone. Initial studies to quantify esterase sequences showed a 5-11-fold increase in resistant aphids compared with susceptible aphids, suggesting the presence of 10-22 gene copies per diploid genome. A more incisive analysis by pulsed-field gel electrophoresis confirmed the presence of about 12 copies of the E4 gene and showed them to be on about 24 kb amplicons, arranged as a tandem array of direct repeats. This, together with previous results from crossing experiments and with recent in situ hybridization studies, confirms that the E4 gene amplification in this aphid clone is heterozygous at a single locus. However, these data show that the gene amplification alone cannot account for the approx. 60 times higher levels of E4 protein and its mRNA present in this aphid clone, and therefore resistance must involve changes in both esterase gene copy number and gene expression.

Changes in the methylation of amplified esterase DNA during loss and reselection of insecticide resistance in peach-potato aphids, Myzus persicae

Insecticide resistance in peach-potato aphids, Myzus persicae, results from the amplification of genes encoding an esterase that hydrolyses and sequesters insecticides. Resistance is normally stable, but highly resistant aphid clones sometimes lose resistance when insecticidal selection pressure is removed. This loss of resistance, termed reversion, arises from a loss of elevated esterase enzyme through transcriptional control, i.e. without loss of the amplified esterase DNA sequences. We have shown that loss of the elevated enzyme occurred simultaneously with loss of methylation at CCGG sites in the amplified DNA sequences. During reselection of resistance in these revertant clones, enzyme levels increased, but there was no corresponding return of methylation to DNA sequences. Thus, although DNA methylation is closely correlated with expression of the amplified esterase genes during reversion, it may not be a factor in the reverse process.

Cloning and sequencing of a cDNA encoding acetylcholinesterase in Colorado potato beetle, Leptinotarsa decemlineata (Say)

A cDNA encoding acetylcholinesterase (AChE, EC 1.1.1.7) was cloned from a cDNA library constructed from an insecticide-susceptible strain of Colorado potato beetle, Leptinotarsa decemlineata (Say). The complete amino acid sequence of AChE deduced from the cDNA consisted of 29 residues for the putative signal peptide and 600 residues for the mature protein with a predicted molecular weight of 67,994. Northern blot analysis of poly(A) RNA showed an approx 13.1-kb transcript. The mature protein sequence had 57 and 61% of amino acid residues identical to those of Drosophila melanogaster and Anopheles stephensi, respectively, and produced a remarkably similar hydropathy profile when compared to those of the two dipterous species. The three residues (Ser, Glu and His) that putatively form the catalytic triad and the six Cys that form intra-subunit disulfide bonds were completely conserved when compared to the other seven AChEs from a broad range of animal species reported to date. Other properties of the deduced protein of AChE, including molecular weight and amino acid composition, agreed well with those of a previously reported study on the purified AChE from the same insect species. All these features firmly established that the cloned cDNA encodes AChE in Colorado potato beetle.

Mosquito carboxylesterase Est alpha 2(1) (A2). Cloning and sequence of the full-length cDNA for a major insecticide resistance gene worldwide in the mosquito Culex quinquefasciatus

Organophosphorus insecticide resistance in Culex mosquitoes is commonly caused by increased activity of one or more esterases. The commonest phenotype involves elevation of the esterases Est alpha 2 (A2) and Est beta 2 (B2). A cDNA encoding the Est alpha 2 esterase has now been isolated from a Sri Lankan insecticide-resistant mosquito (Culex quinquefasciatus, Say) expression library. In line with a recently suggested nomenclature system (Karunaratne, S. H. P. P. (1994) Characterization of Multiple Variants of Carboxylesterases Which Are Involved in Insecticide Resistance in the Mosquito Culex quinquefasciatus. Ph.D. thesis, University of London), as the first sequenced variant of this esterase, it is now referred to as Est alpha 2(1). The full-length cDNA of est alpha 2(1) codes for a 540-amino acid protein, which has high homology with other esterases and lipases and belongs to the serine or B-esterase enzyme family. The predicted secondary structure of Est alpha 2(1) is similar to the consensus secondary structure of proteins within the esterase/lipase family where the secondary and tertiary structures have been resolved. The level of identity (approximately 47% at the amino acid level) between the est alpha 2(1) and the various Culex est beta (B1 and B2) cDNA alleles that have been cloned and sequenced suggests that the two esterase loci are closely related and arose originally from duplication of a common ancestral gene. The lack of a distinct hydrophobic signal sequence for Est alpha 2(1) and two possible N-linked glycosylation sites, both situated close to the active site serine, suggest that it is a nonglycosylated protein that is not exported from the cell. Southern and dot blot analysis of genomic DNA from various insecticide-resistant and susceptible mosquito strains show that the est alpha 2(1) gene, like est beta 2(1), is amplified in resistant strains. The restriction fragment length polymorphism patterns, after probing Southern blots of EcoRI-digested genomic DNA with esta alpha 2(1) cDNA, show that the amplified and nonamplified est alpha alleles differ in the resistant and susceptible Sri Lankan mosquitoes.

The independent gene amplification of electrophoretically indistinguishable B esterases from the insecticide-resistant mosquito Culex quinquefasciatus

Resistance to organophosphates in Culex mosquitoes is typically associated with increased activity of non-specific esterases. The commonest phenotype involves two elevated esterases, A2 and B2, while some strains have elevation of esterase B1 alone. Overexpression of the two B esterase electromorphs is due to gene amplification. Full-length cDNAs coding for amplified esterase B genes from a resistant Cuban strain (MRES, with amplified B1 esterase) and a Sri Lankan strain (PelRR, with amplified B2 esterase) of C. quinquefasciatus have been sequenced. In addition, a partial-length cDNA coding for a B esterase from an insecticide-susceptible Sri Lankan strain (PelSS) has been sequenced. All the nucleotide sequences and the inferred amino acid sequences show a high level of identify (> 95% at the nucleotide and amino acid level), confirming that they are an allelic series. The two B1 esterase nucleotide sequences (MRES and the previously published TEM-R [Mouches, Pauplin, Agarwal, Lemieux, Herzog, Abadon, Beyssat-Arnaouty, Hyrien, De Saint Vincent, Georghiou and Pasteur (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 2574-2578]) showed the lowest identity, and restriction-fragment-length-polymorphism analysis of the two strains was different. On the basis of these data we suggest that the two electrophoretically identical B1 esterase isoenzymes from California and Cuba have been amplified independently. Alternatively, if amplification has occurred only once, the original amplification has not occurred recently.

The peach-potato aphid Myzus persicae and the tobacco aphid Myzus nicotianae have the same esterase-based mechanisms of insecticide resistance

Biochemical and molecular studies have established that in the peach-potato aphid, Myzus persicae, insecticide resistance is conferred by amplification of genes encoding the insecticide-detoxifying esterases E4 or FE4. Here we report that two insecticide-resistant clones of the closely related tobacco aphid Myzus nicotianae have elevated esterases indistinguishable from E4 and FE4 and amplified esterase DNA sequences, and flanking regions, with identical restriction maps to the M. persicae genes. Furthermore, the DNA sequences of c. 630 bp fragments of the E4 and FE4 genes of M. persicae are different from each other but identical to the fragment from corresponding M. nicotianae clones. The existence of apparently identical insecticide resistance genes in the two species can be best explained by the selection of the amplified genes in M. persicae, transfer to hybrids of M. persicae and M. nicotianae by sexual reproduction and subsequent spread through M. nicotianae populations.

Evolutionary genetics of Drosophila esterases

Over 30 carboxylester hydrolases have been identified in D. melanogaster. Most are classified as acetyl, carboxyl or cholinesterases. Sequence similarities among most of the carboxyl and all the cholinesterases so far characterised from D. melanogaster and other eukaryotes justify recognition of a carboxyl/cholinesterase multigene family. This family shows minimal sequence similarities with other esterases but crystallographic data for a few non-drosophilid enzymes show that the family shares a distinctive overall structure with some other carboxyl and aryl esterases, so they are all put in one superfamily of/beta hydrolases. Fifteen esterase genes have been mapped in D. melanogaster and twelve are clustered at two chromosomal sites. The constitution of each cluster varies across Drosophila species but two carboxyl esterases in one cluster are sufficiently conserved that their homologues can be identified among enzymes conferring insecticide resistance in other Diptera. Sequence differences between two other esterases, the EST6 carboxyl esterase and acetylcholinesterase, have been interpreted against the consensus super-secondary structure for the carboxyl/cholinesterase multigene family; their sequence differences are widely dispersed across the structure and include substantial divergence in substrate binding sites and the active site gorge. This also applies when EST6 is compared across species where differences in its expression indicate a difference in function. However, comparisons within and among species where EST6 expression is conserved show that many aspects of the predicted super-secondary structure are tightly conserved. Two notable exceptions are a pair of polymorphisms in the substrate binding site of the enzyme in D. melanogaster. These polymorphisms are associated with differences in substrate interactions in vitro and demographic data indicate that the alternative forms are not selectively equivalent in vivo.