Nucleotide sequence of a mosquito 18S ribosomal RNA gene

A novel p53 paralogue mediates antioxidant defense of mosquito cells to survive dengue virus replication

Mosquito cells allow dengue viruses (DENVs) to undergo replication without causing serious deleterious effects on the cells, leading to advantages for dissemination to other cells. Despite this, increased accumulation of reactive oxygen species (ROS) is usually detected in C6/36 cells with DENV2 infection as shown in mammalian cells. Uniquely, oxidative stress caused by the ROS is alleviated by eliciting antioxidant defense which leads to protection of mosquito cells from the infection. In the present study, a novel p53 paralogue (p53-2) was identified and proved to be regulated in C6/36 cells with DENV2 infection. With a gene-knockdown technique, p53-2 was demonstrated to transcribe catalase which plays a critical role in reducing ROS accumulation and the death rate of infected cells. Ecologically, a higher survival rate of mosquito cells is a prerequisite for prosperous production of viral progeny, allowing infected mosquitoes to remain healthy and active for virus transmission.

RDNA sequences of Anopheles species from the Iberian Peninsula and an evaluation of the 18S rRNA gene as phylogenetic marker in anophelinae

The complete 18S rDNA and internal transcribed spacer (ITS)-2 rDNA sequences were obtained from Anopheles atroparvus Van Thiel and Anopheles plumbeus Stephens from two areas of Spain. The number of nucleotide differences in the 18S rDNA of the two species is high compared with differences in the same gene of other invertebrate vectors. In Anopheles, short 18S rDNA sequences are richer in AT than the longer sequences, which are richer in GC and include extremely GC-biased expanded regions. Four small regions in the variable regions V4 and V7 contain the majority of nucleotide differences. The results did not support the use of partial sequences for relationship analyses. Genetic distances and phylogenetic analyses supported the most recent classification of Anopheles. The complete 18S rDNA sequence is better for studying anopheline phylogenetics.

Rate of elimination of Wolbachia pipientis by doxycycline in vitro increases following drug withdrawal

Wolbachia pipientis is an obligate intracellular bacterium within the family Anaplasmataceae that infects many terrestrial arthropods and arthropod-transmitted nematodes (filariae). Several filarial species are major human pathogens, and antibiotics with activity against Wolbachia offer a promising new therapeutic approach, since the adult worms are relatively refractory to conventional anthelmintics but depend on Wolbachia for reproduction and viability. In a natural filarial parasite of cattle, Onchocerca ochengi, intermittent chemotherapy is adulticidal whereas the equivalent dose administered as a continuous treatment is not. To investigate this further and to aid the design of efficacious regimens for human therapy, we used Wolbachia-infected Aedes albopictus mosquito cells in vitro. Here, we describe for the first time the accelerated depletion of bacteria after antibiotic withdrawal relative to the rate of elimination in the continuous presence of the drug. Mosquito cells were incubated with doxycycline while changes in 16S (bacterial) and 18S (host) rRNA and rRNA genes were determined by quantitative PCR assays. In cultures treated for 7 or 14 days followed by 7 days of drug withdrawal, the Wolbachia-to-Aedes rRNA ratio declined by approximately 6 log, whereas immediately after 14 or 21 days of continuous treatment, the reduction was only approximately 4 log (P < 0.05). However, low levels of 16S rRNA remained after 21 days of treatment, irrespective of whether doxycycline was withdrawn. Application of similar methodology to related intracellular bacteria may reveal that this posttreatment effect is not restricted to Wolbachia and could have wider implications for the design of intermittent regimens for antibiotic chemotherapy.

Completion of the sequence of the nuclear ribosomal DNA subunit of Simulium sanctipauli, with descriptions of the 18S, 28S genes and the IGS

We describe the IGS-ETS, 18S and 28S ribosomal gene sequences of Simulium sanctipauli Vajime & Dunbar, a member of the S. damnosum Theobald (Diptera: Simuliidae) complex of blackflies (Diptera: Simuliidae). These regions, together with the ITS-1, ITS-2 and 5.8S rDNA presented elsewhere (accession number U36206), constitute the composite sequence of the entire rDNA unit, making S. sanctipauli the second dipteran species of medical importance for which the entire rDNA has been sequenced. Despite the lack of sequence identity, the IGS of S. sanctipauli showed some structural similarities to other Diptera, i.e. the mosquito Aedes albopictus Skuse (Culicidae), the fruitfly Drosophila melanogaster Meigen (Drosophilidae) and the tsetse Glossina (Glossinidae). Two blocks of tandemly repeated subunits were present in the IGS of S. sanctipauli and, unlike other species of Diptera, they contained no duplications of promoter-like sequences. However, two promoter-like sequences were identified in the unique DNA stretches of the IGS by their sequence similarity to the promoter of Aedes aegypti L. (Diptera: Culicidae). The observed sequence variation can be explained, as in the case of Drosophila spp., by the occurrence of slippage-like and point mutation processes, with unequal crossing-over homogenizing (to a certain extent) the region throughout the gene family and blackfly population. The 18S and 28S rDNA genes show more intraspecific variability within the expansion segments than in the core regions. This is also the case in the interspecific comparison of these genes from S. sanctipauli with those of Simulium vittatum, Ae. albopictus and D. melanogaster. This pattern is typical of many eukaryotes and likely to be the result of a more relaxed functional selection in the expansion segments than on the core regions. The A + T content of the S. sanctipauli genes is high and similar to those of other Diptera. This could be the result of a change in the mutation pressure towards AT in the Diptera lineage.

Differential regulation of ribosomal protein gene expression in Aedes aegypti mosquitoes before and after the blood meal

In fat body of the mosquito, Aedes aegypti, a cycle of ribosome accumulation and degradation accompanies synthesis of the yolk protein precursor, vitellogenin. Here we compare the transcription and translation of ribosomal proteins rpS6, rpL8 and rpL34, relative to rRNA and vitellogenin genes in Aedes aegypti fat body after eclosion, and in response to a blood meal. Analysis using Northern blots and reverse-transcription polymerase chain reactions (RT-PCR) showed that the rpS6, rpL8 and rpL34 genes are coordinately regulated with respect to one another, and that ribosomal protein gene expression in this system was predominantly regulated by transcription during the 3-4 days between adult eclosion and blood feeding. After a blood meal, ribosomal protein mRNA levels remained similar to those in unfed females during the first 18 h, then declined to minimum levels by 48 h after the blood meal. These data indicate that transcription of ribosomal protein genes is low in vitellogenic mosquitoes, relative to previtellogenic females. Experiments with the dissected fat body, however, showed that levels of acetic acid-soluble proteins increased by approximately threefold between 12 and 24 h after the blood meal. Taken together, these observations suggest that the active translation of ribosomal proteins from stable mRNA accompanies ribosome biosynthesis after the blood meal. Thus, in the fat body of adult female mosquitoes, the expression of ribosomal protein genes is regulated at the level of transcription before the blood meal, while translational control is the predominant regulatory mechanism after the blood meal.

Polyamines, and effects from reducing their synthesis during egg development in the yellow fever mosquito, Aedes aegypti

Development of eggs after a blood meal in the yellow fever mosquito Aedes aegypti involves hormonal changes, synthesis of nucleic acids, activation of the digestive enzyme trypsin, and production of the yolk protein vitellogenin. Polyamines have been implicated in growth processes and were here examined for possible involvement during egg development. The data suggest that polyamines are important for normal vitellogenesis in the mosquito. Polyamine levels and activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase, key enzymes in the polyamine pathway, were determined in the fat body for two days after a blood meal. During the time that the macromolecules required for vitellogenesis were being synthesized, polyamine levels increased as did the activities of their rate-limiting enzymes. Administration of suicide inhibitors of ornithine decarboxylase, alpha-difluoromethylornithine (DFMO) and alpha-monofluoromethyldehydroornithine methylester (MDME), limited increased polyamine levels and disrupted macromolecular syntheses, particularly during the first twenty-four hours after blood feeding. Specific metabolic processes reduced by DFMO included trypsin activity, and production of RNA, DNA and vitellogenin. MDME had differential effects on transcription of some mRNA species made after an oogenic meal. The level of actin mRNA was not affected by inhibiting polyamine synthesis, but the mRNA levels of vitellogenin, trypsin, and the vitelline membrane protein were decreased. Adding polyamines to a meal containing DFMO or MDME partially reversed the effects of these inhibitors. Increases in spermidine and spermine were associated with these reversals.

Characterization of a carboxypeptidase A gene from the mosquito, Aedes aegypti

A gut-specific carboxypeptidase A gene (AeCPA) from the mosquito, Aedes aegypti, was cloned and characterized. The gene has an open reading frame that predicts a protein of 427 amino acids, 61% of which are identical to an Anopheles gambiae carboxypeptidase A sequence. AeCPA messenger RNA (mRNA) was not detected during larval and pupal development. In situ hybridization experiments indicated that AeCPA mRNA is expressed by posterior midgut epithelial cells. In sharp contrast to An. gambiae carboxypeptidase A gene expression, AeCPA mRNA accumulates to high levels only late ( approximately 16-24 h) after ingestion of a blood meal. The temporal profile of AeCPA gene induction is similar to that of Ae. aegypti late trypsin, suggesting the existence of common regulatory elements.

The ribosomal protein L34 gene from the mosquito, Aedes albopictus: exon-intron organization, copy number, and potential regulatory elements

We describe the structural analysis of genomic DNA encoding ribosomal protein (rp) L34 from the mosquito, Aedes albopictus. Comparison of genomic DNA sequences encompassing approximately 8 kb with the rpL34 cDNA sequence showed that the gene contains three exons and two introns, encoding a primary transcript with a deduced size of 6196 nucleotides from the transcription start site to the polyadenylation site. Exon 1, which is not translated, measures only 45 bp, and is separated from Exon 2 by a 359 bp intron. Exon 2 measures 78 bp, and contains the AUG translation initiation codon 14 nucleotides downstream of its 5'-end. Downstream of Exon 2 is a 5270 bp intron, followed by the remainder of the coding sequence in Exon 3, which measures 444 bp including the polyadenylation signal. We used a novel PCR-based procedure to obtain 1.7 kb of DNA upstream of the rpL34 gene. Like the previously described Ae. albopictus rpL8 gene and various mammalian rp genes, the DNA immediately upstream of the rpL34 gene lacks the TATA box, and the rpL34 transcription initiation site is embedded in a characteristic polypyrimidine tract. The 5'-flanking DNA contained a number of cis-acting elements that potentially interact with transcription factors characterized by basic domains, zinc-coordinating DNA binding domains, helix-turn-helix motifs, and beta scaffold factors with minor groove contacts. Particularly striking was the conservation of an AP-4 binding site within 100 nucleotides upstream of the transcription initiation site in both Aal-rpL34 and Aal-rpL8 genes. Comparison of Southern hybridization signals using probes from the 5' and 3'-ends of the 5.3 kb second intron and the cDNA suggested that the Ae. albopictus rpL34 gene most likely occurs as a single expressed copy per haploid genome with restriction enzyme polymorphisms in the upstream flanking DNA and the likely presence of one or more pseudogenes.

Changes in ribosomal protein rpL8 mRNA during the reproductive cycle of the mosquito, Aedes aegypti

Changes in the abundance of ribosomal protein rpL8 mRNA were compared with 18S rRNA and vitellogenin mRNA in fat body from adult female Aedes aegypti during the reproductive cycle. Levels of rpL8 mRNA began to increase within 2 h after adult eclosion, peaked at about 24 h post-eclosion, and remained high throughout the next 48 h. During this same period, rRNA abundance increased about 2-fold. After the bloodmeal, levels of rpL8 mRNA gradually decreased over the next 48 h, while rRNA levels increased about 4-fold within 16-24 h post-bloodfeeding and eventually returned to previtellogenic levels. Vitellogenin mRNA was induced only after the bloodmeal, and disappeared by 48 h after feeding. After oviposition, rpL8 mRNA levels again increased to pre-bloodfeeding levels. These results indicate that rpL8 mRNA transcription in mosquito fat body is independent of rRNA transcription during the previtellogenic acquisition of competence and also during the post-bloodmeal ovarian cycle. Moreover, unlike the vitellogenin gene, the rpL8 gene is under post-transcriptional regulation in blood-fed females.

Promoter utilization in a mosquito ribosomal DNA cistron

In the mosquito Aedes albopictus, two potential RNA polymerase I promoters that map 531 and 143 nucleotides upstream of the 18S rRNA gene have been defined on the basis of sequence homology with rRNA promoters from other species. Using the polymerase chain reaction, we confirmed that a 717 nucleotide region spanning the upstream (-531) and downstream (-143) promoters is homogeneous in genomic DNA and in cloned DNA. DNA probes representing each of these promoters, as well as upstream "spacer" promoters, exhibited protein-binding activity, and each unlabeled probe was an effective competitor of protein binding with the other probes, suggesting that these potential regulatory sequences interact with a common protein(s). Analysis of precursor ribosomal RNAs accumulated during temperature shock indicated that transcription is initiated primarily at the upstream (-531) promoter. RNAse protection and primer extension analyses confirmed the predominant use of this promoter, both in cultured cells and in mosquito life stages.

Evidence for DNA endonuclease activity in nuclear extracts from mosquito cells

We describe a deoxyribonuclease activity from nuclear protein extracts of cultured Aedes albopictus mosquito cells. The nuclease cleaved linear and circular double-stranded DNA, first generating 3' OH single-stranded nicks followed by second strand cleavage, but had little or no exonucleolytic activity. Detection of this activity was optimal at pH 7.1, in the presence of a divalent cation (Mg2+, Ca2+, Mn2+, Ba2+). In the presence of Mg2+, Zn2+, Hg2+ and Cu2+ inhibited activity, sulfhydryl reagents and ATP had no effect. At physiological temperatures (18-35 degrees C), linear double-stranded DNA probes were preferentially cleaved near sites containing 3-6 consecutive deoxyadenine/thymine base pairs. Results from salt dependency and drug inhibition studies, combined with inspection of DNA sequence, suggested that DNA structure is among the parameters that determine preferred cleavage sites.

Molecular organization and evolution of mosquito genomes

Given the importance of mosquitoes as disease vectors, relatively little is known about the molecular organization and evolution of mosquito genomes as compared to other insects such as fruit flies. The advances in recombinant DNA technology and the possibility that mosquito populations could be controlled and genetically manipulated using such technology has stimulated considerable research during the last few years in the areas of genome organization and evolution, genome mapping, endogenous transposable elements, and mapping and characterization of genes conferring susceptibility to different parasites and pathogens. This review summarizes research currently underway in our laboratory and elsewhere in these areas.

Congruent evolution between whiteflies (Homoptera: Aleyrodidae) and their bacterial endosymbionts based on respective 18S and 16S rDNAs

Whiteflies (family Aleyrodidae) possess heritable eubacterial endosymbionts sustained in specialized organ-like structure called mycetomes. Comparisons of distances between the ash whitefly, Siphoninus phillyreae, and two biotypes ("A" and "B") of the sweetpotato whitefly, Bemisia tabaci, based on sequence analysis of genes for 18S rRNAs (rDNAs), were equivalent to the distances represented by the 16S rDNAs of their respective endosymbionts. This finding indicates that evolutionary divergence in whitefly hosts and their endosymbionts is congruent. The nucleotide sequences of the 18S rDNAs and endosymbiont 16S rDNAs indicate the two biotypes of B. tabaci are the same species.

Sequence and secondary structure comparisons of ITS rDNA in mosquitoes (Diptera: Culicidae)

Sequences of the internal transcribed spacers (ITS1 and ITS2) of the mosquito Aedes aegypti, and the ITS2 of six related species, A. simpsoni, A. albopictus, A. vexans, A. triseriatus, Haemagogus mesodentatus, and Psorophora ferox are reported. Intraspecific variation in A. aegypti ITS1 is 1.07% among four clones from three individuals, and in the ITS2 is 1.17% among 15 clones from four individuals. In A. simpsoni, intraspecific ITS2 variation is 0.46% among 10 clones from a single individual. Alignment of the ITS2 sequence of the seven species reveals several homologous domains. Secondary structure predictions for the ITS2 region indicate that these domains base pair to form a core region central to several stem features. The sequence outside the ITS2 homologous domains tends to be GC-rich and characteristically slippage generated; these areas preserve or add to the stem length of the predicted secondary structures. These ITS2 intraspacer variable regions resemble previously described expansion segments of the 28S gene region. Evolutionary analysis of the ITS2 of these species, using both sequence and secondary structure information, leads to the prediction of divergence in the mosquito tribe Aedini that is not clearly reflected in current taxonomic designations.

Primary structure of the ribosomal DNA intergenic spacer from the mosquito, Aedes albopictus

We have determined the primary structure of a 4.7-kb portion of the ribosomal DNA intergenic spacer from cultured cells of the mosquito, Aedes albopictus. Immediately upstream from the 18S rRNA gene was a 753-bp sequence containing two regions similar to known RNA polymerase I promoters, each preceded by potential transcription termination signals. Upstream from this putative promoter region was a 3.15-kb tandem array of 17 direct repeats with a consensus sequence length of 201 bp. The 201-bp repeats contained imperfect antisense duplications of 11-bp core domain regions in the putative RNA polymerase I promoters, and sequences of possible significance in recombination. Farthest upstream of the 18S rRNA gene was an 803-bp region containing two copies each of 34-, 48-, and 64-bp elements separated by apparently unique sequence. This first detailed structural analysis of a ribosomal DNA intergenic spacer from a member of the lower Diptera has revealed features similar to those described for the higher Diptera as well as conserved motifs presumably critical to rRNA transcription.