Isolation and characterization of cDNA encoding Argonaute, a component of RNA silencing in shrimp (Penaeus monodon).

We have identified a cDNA clone that encodes a protein with high sequence homology to Argonaute proteins of mammals and Drosophila melanogaster. The cDNA of Penaeus monodon (Pm Ago) consisted of 3178 nucleotides encoding 939-amino acid residues with a calculated molecular weight of 104 kDa. The primary structure of Pm Ago showed the presence of two signature domains, PAZ and PIWI domains that exhibit highest homology to their counterparts in D. melanogaster. The inferred protein sequence of Pm Ago was 80.8% identical with D. melanogaster and 82.1% identical with Anopheles gambiae Ago proteins. Phylogenetic analysis of Pm Ago with other invertebrate and vertebrate Argonaute proteins suggested that Pm Ago belongs to the Ago1 subfamily that plays crucial roles in stem cell differentiation or RNA interference (RNAi). Semi-quantitative RT-PCR analysis showed that the gene is highly expressed in the lymphoid organ and moderately expressed in intestine, muscle, pleopods and hemocytes. The expression of Pm Ago1 mRNA was 2-3-fold increased during the early period of viral infection but declined rapidly at 30 hour post infection. By contrast, infection of shrimp by a bacterial pathogen, Vibrio harveyi did not induce a reduction of Pm Ago1 mRNA suggesting that its expression is associated with virus infection.

Structural and antigenic analysis of the yellow head virus nucleocapsid protein p20.

Yellow head virus (YHV) is an invertebrate nidovirus that is highly pathogenic for marine shrimp. Nucleotide sequence analysis indicated that the YHV ORF2 gene encodes a basic protein (pI = 9.9) of 146 amino acids with a predicted molecular weight of 16,325.5 Da. The deduced amino acid sequence indicated a predominance of basic (15.1%), acidic (9.6%) and hydrophilic polar (34.3%) residues and a high proportion proline and glycine residues (16.4%). The ORF2 gene was cloned and expressed in Escherichia coli as a M(r) = 21 kDa His(6)-protein that reacted with YHV nucleoprotein (p20) monoclonal antibody. Segments representing the four linear quadrants of the nucleoprotein were also expressed in E. coli as GST-fusion proteins. Immunoblot analysis using YHV polyclonal rabbit antiserum indicated the presence of linear epitopes in all except the V(37)-Q(74) quadrant. Immunoblot analysis of the GST-fusion proteins and C-terminally truncated segments of the nucleoprotein allowed mapping of YHV monoclonal antibodies Y19, Y20 and YII4 to linear epitopes in the acidic domain between amino acids I(116) and E(137). The full-length nucleoprotein was expressed at high level in E. coli and was easily purified in quantity from the soluble cell fraction by Ni(+)-NTA affinity chromatography.

Complete nucleotide sequence and genomic organization of hepatopancreatic parvovirus (HPV) of Penaeus monodon.

We have determined the genome of hepatopancreatic parvovirus (HPV), a minus, single-stranded DNA virus isolated from infected Penaeus monodon in Thailand. Its genome consisted of 6321 nucleotides, representing three large open reading frames (ORFs) and two non-coding termini. The left (ORF1), mid (ORF2), and right (ORF3) ORFs on the complementary (plus) strand may code for 428, 579, and 818 amino acids, equivalent to 50, 68, and 92 kDa, respectively. The 5' and 3' ends of viral genome contained hairpin-like structure length of approximately 222 and 215 bp, respectively. No inverted terminal repeat (ITR) was detected. The ORF2 contained conserved replication initiator motif, NTP-binding and helicase domain similar to NS-1 of other parvoviruses. Therefore, it most likely encoded the major nonstructural protein (NS-1). The ORF1 encoded putative nonstructural protein-2 (NS-2) with unknown function. The ORF3 of the HPV genome encoded a capsid protein (VP) of approximately 92 kDa. This may be later cleaved after arginine residue to produce a 57-kDa structural protein. A phylogenetic tree based on conserved amino acid sequences (119 aa) revealed that it is closely related to Brevidensoviruses, which are shrimp parvovirus (IHHNV) and mosquito densoviruses (AaeDNV and AalDNV). However, the overall genomic organization and genome size of HPV were different from these parvoviruses, for instance, the non-overlapping of NS1 and NS2, the larger VP gene, and the bigger genome size. This suggested that this HPV virus is a new type in Parvoviridae family. We therefore propose to rename this virus P. monodon densovirus (PmDNV).

Target for standard Thai PCR assay identical in 12 white spot syndrome virus (WSSV) types that differ in DNA multiple repeat length.

A Thai PCR detection method (WSSV-232) yielding a 232 bp amplicon has been used for detection of white spot syndrome virus (WSSV) since 1996. It targets ORF 91 in the full sequence of the only Thai WSSV isolate at GenBank (AF369029). At the beginning of 2002, some Thai shrimp farmers complained that ponds stocked with WSSV-232 PCR negative post-larvae (PL) later suffered WSSV disease outbreaks. Although these outbreaks may have resulted from horizontal transmission of WSSV after stocking, it was also possible that they resulted from false negative PCR test results due to genetic changes at the PCR-assay target after the first appearance of WSSV in Thailand in 1995. Indeed, recent results have revealed at least 12 WSSV variants in Thailand that can be distinguished based on differences in DNA multiple repeat lengths in ORF 94 (GenBank AF369029). To test for variation in the WSSV-232 target sequence in ORF 91, 20 DNA extracts derived from field samples and representing 9 of the WSSV DNA multiple repeat groups were subjected to PCR amplification and sequencing using primers that generated a 403 bp amplicon covering the target for the WSSV-232 assay. An additional three repeat types were included from archived material. Analysis revealed that the 232 bp target sequence in ORF 91 was unchanged in all of the 12 types tested and that the original WSSV-232 detection system was still valid. Thus, any false negative PCR test results leading to farmer complaints would probably have arisen from small sample sizes and low sensitivity of the single-step PCR assay. If so, false negative results could be reduced by the use of nested PCR assays with larger PL sample sizes. .

Involvement of specific proteins (Sp1/Sp3) and nuclear factor Y in basal transcription of the distal promoter of the rat pyruvate carboxylase gene in beta-cells.

Pyruvate carboxylase plays diverse roles in different biosynthetic pathways, including glucose-induced insulin secretion in pancreatic beta-cells. We have localized the control region of the P2 promoter by generating a series of 5'-nested deletion constructs, and both 25- and 9-bp internal deletion constructs, as well as by performing site-directed mutagenesis. Transient transfections of these constructs into INS-1 cells identified a CCAAT box and a GC box that are located at -65/-61 and -48/-41, respectively, as the important determinants. Disruption of the GC box resulted in a 4-fold reduction of the reporter activity, while disruption of the proximal CCAAT box (-65/-61) but not the distal CCAAT box (-95/-91) increased the reporter activity by 3-fold. Simultaneous disruptions of both the GC box and the CCAAT box reduced the reporter activity to a level that was close to that of the single GC box mutation. Electrophoretic mobility shift assays (EMSAs) and supershift EMSAs using nuclear extract from INS-1 cells demonstrated that Sp1 and Sp3 bind a GC box while the nuclear factor Y was shown to bind the proximal but not the distal CCAAT box.

RT-PCR detection of yellow head virus (YHV) infection in Penaeus monodon using dried haemolymph spots.

Sample collection and RNA isolation from shrimp haemolymph for RT-PCR diagnosis of yellow head virus (YHV) infections is crucial for disease control programs for cultivated shrimp in Thailand. Problems with RNA degradation arise when field samples must be collected far from the laboratory by relatively inexperienced personnel who do not have ready access to sophisticated reagents. In an attempt to solve this problem, haemolymph samples from shrimp were collected either by mixing with 10% (w/v) sodium citrate or by spotting on ISOCODE filter paper. RNA was extracted subsequently either by a rapid boiling method or by using TRI reagent and the extracts were used in a semi-quantitative, non-stop, semi-nested RT-PCR assay for YHV. Dried haemolymph spots on ISOCODE filter paper extracted with TRI reagent gave the most reliable and reproducible results. It also allowed longer periods of storage at room temperature.

Multiplex RT-nested PCR differentiation of gill-associated virus (Australia) from yellow head virus (Thailand) of Penaeus monodon.

A multiplex RT-nested PCR has been developed to detect and differentiate the closely related prawn viruses, gill-associated virus (GAV) from Australia and yellow head virus (YHV) from Thailand. RT-PCR using primers to conserved sequences in the ORF1b gene amplified a 794 bp region of either GAV or YHV. Nested PCR using a conserved sense primer and either a GAV- or YHV-specific antisense primer to a divergent sequence differentially amplified a 277 bp region of the primary PCR amplicon. Multiplexing the YHV antisense primer with a GAV antisense primer to another divergent sequence allowed the viruses to be distinguished in a single nested PCR. Nested PCR enhanced detection sensitivity between 100- and 1000-fold and GAV or YHV RNA was detectable in approximately 10 fg lymphoid organ total RNA. The multiplex RT-nested PCR was also able to co-detect GAV and YHV RNA mixed over a wide range of concentrations to simulate potential dual-infection states. The robustness of the test was examined using RNA samples from Penaeus monodon prawns infected either chronically or acutely with GAV or YHV and collected at different locations in Eastern Australia and Thailand between 1994 and 1998. GAV- (406 bp) or YHV-specific (277 bp) amplicons were differentially generated in all cases, including five YHV RNA samples in which no primary RT-PCR amplicon was detected. Sequence analysis of GAV and YHV PCR amplicons identified minor variations in the regions targeted by the virus-specific antisense primers. However, none occurred at positions that critically affected the PCR.

Expression of human pyruvate carboxylase in insect cells using the baculovirus system.

Constructs containing cDNA encoding human pyruvate carboxylase (PC) with and without a hexahistidine (6x His) tag at the N-terminal of the mature enzyme have been cloned under the control of the polyhedrin promoter. These two constructs were co-transfected with the baculovirus genome into Sf9 cells to produce recombinant baculoviruses harbouring human PC cDNA. The expression of human PC under the control of the polyhedrin promoter was found to be at its highest level at 4 days post-infection. The expressed material accounted for up to 70% of total cellular protein with 5% of this expressed material being found in the soluble fraction. The recombinant human 6x His-PC isolated with a purity of approximately 50% using a Ni-NTA agarose column was found to have the specific activity of 7U/mg, which was similar to that produced from a 293T stable line [Biochem. Biophys. Res. Commun. 266 (1999) 512]. This is the first report of a heterologous expression system for recombinant human PC.

High variation in repetitive DNA fragment length for white spot syndrome virus (WSSV) isolates in Thailand.

White spot syndrome virus (WSSV) presently causes the most serious losses to shrimp farmers worldwide. Earlier reports of high DNA sequence homology among isolates from widely separated geographical regions suggested that a single virus was the cause. However, we have found surprisingly high variation in the number of 54 bp DNA repeats in ORF94 (GenBank AF369029) from 55 shrimp ponds (65 shrimp samples) experiencing WSSV outbreaks in Thailand in 2000 and 2002. These were detected by PCR amplification using primers ORF94-F and ORF94-R flanking the repeat region. Altogether, 12 different repeat groups were found (from 6 to 20 repeats) with 8 repeats being most frequent (about 32%). Extracts prepared from individual shrimp in the same outbreak pond belonged to the same repeat group while those collected at the same time from separate WSSV outbreak ponds, or from the same ponds at different times, usually belonged to different repeat groups. This suggested that different outbreaks were caused by different WSSV isolates. In contrast to the highly variable numbers of repeats, sequence variation within the repeat region was confined to either T or G at Position 36. These variations may be useful for epidemiological studies on the local and global movement of WSSV, since there is high variation in the number of repeats (good for local studies) but little sequence change (good for global studies).

Seasonal variation in white spot syndrome virus-positive samples in broodstock and post-larvae of Penaeus monodon in Thailand.

Records of PCR test results for white spot syndrome virus (WSSV) were reviewed in Thailand from 1998 to 2000 for wild Penaeus monodon broodstock purchased by hatcheries and for post-larvae (PL) brought by farmers to diagnostic laboratories for testing. Samples for PCR comprised DNA extracts from the last pleopod dissected from broodstock females after the first spawning and DNA extracts from whole, homogenized PL. There was a consistent pattern of fluctuation in percentage of WSSV-positive broodstock and PL. In broodstock, the fluctuation pattern was similar each year, with a low percentage (0 to 6%) from January to May and a higher percentage (6 to 18%) for the rest of the year, with a peak from September to November. The fluctuation pattern for PL was similar but offset with peaks and troughs occurring approximately 2 mo after those for the broodstock. The peak percentages of broodstock-positive samples were roughly constant from year to year, but those for PL decreased progressively in magnitude from 1998 to 2000. Examination of a small number of hatcheries in 2000 revealed that the percentage of WSSV-positive PL samples was significantly lower for hatcheries that routinely discarded WSSV-PCR-positive wild broodstock when compared to hatcheries that did not.

Identification and analysis of gp116 and gp64 structural glycoproteins of yellow head nidovirus of Penaeus monodon shrimp.

Yellow head virus (YHV) is a major agent of disease in farmed penaeid shrimp. YHV virions purified from infected shrimp contain three major structural proteins of molecular mass 116 kDa (gp116), 64 kDa (gp64) and 20 kDa (p20). Two different staining methods indicated that the gp116 and gp64 proteins are glycosylated. Here we report the complete nucleotide sequence of ORF3, which encodes a polypeptide of 1666 amino acids with a calculated molecular mass of 185 713 Da (pI=6.68). Hydropathy analysis of the deduced ORF3 protein sequence identified six potential transmembrane helices and three ectodomains containing multiple sites for potential N-linked and O-linked glycosylation. N-terminal sequence analysis of mature gp116 and gp64 proteins indicated that each was derived from ORF3 by proteolytic cleavage of the polyprotein between residues Ala(228) and Thr(229), and Ala(1127) and Leu(1128), located at the C-terminal side of transmembrane helices 3 and 5, respectively. Comparison with the deduced ORF3 protein sequence of Australian gill-associated virus (GAV) indicated 83 % amino acid identity in gp64 and 71 % identity in gp116, which featured two significant sequence deletions near the N terminus. Database searches revealed no significant homology with other proteins. Recombinant gp64 expressed in E. coli with and without the C-terminal transmembrane region was shown to react with antibody raised against native gp64 purified from virions.

Detection of hepatopancreatic parvovirus in Thai shrimp Penaeus monodon by in situ hybridization, dot blot hybridization and PCR amplification.

Hepatopancreatic parvovirus (HPV) infects the hepatopancreas in penaeid shrimp and retards their growth. The DNA sequence of HPV from Thai shrimp Penaeus monodon (HPVmon) differs from HPV of Penaeus chinensis (HPVchin) by approximately 30%. In spite of this difference, commercial PCR primers (DiagXotics) developed from HPVchin to yield a 350 bp PCR product do give a 732 bp product with HPVmon DNA template. On the other hand, the sensitivity of HPVmon detection with these primers and with hybridization probes designed for HPVchin is significantly lower than it is with HPVchin. To improve sensitivity for HPVmon detection, we used the sequence of the 732 bp HPVmon PCR amplicon described above to develop specific PCR primers (H441F and H441R) and hybridization probe. The primers could detect as little as 1 fg of purified HPVmon DNA while the 441 bp digoxygenin-labeled PCR product gave strong, specific reactions with in situ hybridization and with hybridization blots. In contrast, negative results were obtained using DNA from all other pathogens tested and from DNA of P. monodon. Supernatant solution from boiled, fresh shrimp fecal and postlarval samples homogenized in 0.025% NaOH/0.0125% SDS could be used to detect as little as 0.1 pg HPVmon DNA by the PCR reaction. By dot blot hybridization, a visible signal was obtained with purified HPVmon DNA at 0.01 pg, but detection in spiked feces and postlarval samples was only 1 and 0.1 pg, respectively.