Strategy to screen long DNA inserts in Escherichia coli

Phylogenetic position and replication kinetics of Heliothis virescens ascovirus 3h (HvAV-3h) isolated from Spodoptera exigua

Insect-specific ascoviruses with a circular genome are distributed in the USA, France, Australia and Indonesia. Here, we report the first ascovirus isolation from Spodoptera exigua in Hunan, China. DNA-DNA hybridization to published ascoviruses demonstrated that the new China ascovirus isolate is a variant of Heliothis virescens ascovirus 3a (HvAV-3a), thus named HvAV-3h. We investigated the phylogenetic position, cell infection, vesicle production and viral DNA replication kinetics of HvAV-3h, as well as its host-ranges. The major capsid protein (MCP) gene and the delta DNA polymerase (DNA po1) gene of HvAV-3h were sequenced and compared with the available ascovirus isolates for phylogenetic analysis. This shows a close relationship with HvAV-3g, originally isolated from Indonesia, HvAV-3e from Australia and HvAV-3c from United States. HvAV-3h infection induced vesicle production in the SeE1 cells derived from S. exigua and Sf9 cells derived from S. frugiperda, resulting in more vesicles generated in Sf9 than SeE1. Viral DNA replication kinetics of HvAV-3h also demonstrated a difference between the two cell lines tested. HvAV-3h could readily infect three important insect pests Helicoverpa armigera (Hübner), Spodoptera exigua (Hübner) and Spodoptera litura (Fabricius) from two genera in different subfamilies with high mortalities.

Transcriptional analysis of a major capsid protein gene from Spodoptera exigua ascovirus 5a

The major capsid protein (mcp) gene of Spodoptera exigua ascovirus 5a (SeAV-5a) was confirmed by aphidicolin viral DNA replication inhibition analysis to be a late gene. The 5' and 3' ends of mcp gene transcripts have been mapped. Primer extension analyses indicated that transcription of the mcp gene initiates from a cytosine 25 nucleotides (nt) upstream of the translation start codon. Two independent approaches by 3' rapid amplification of cDNA ends (3' RACE) and oligo (dT) cellulose binding assay suggested that SeAV-5a mcp mRNA is polyadenylated. Analyses by 3' RACE also revealed that mcp transcripts terminate at a U, either at 26 or 38 nt downstream of the translation stop codon. The putative 5' transcription control region of the SeAV-5a mcp gene shares similarities with other ascoviruses and Chilo iridescent virus (CIV), containing a conserved TATA-box-like motif (TAATTAAA) and an ATTTGATCTT motif upstream of it. The 3' downstream regions of the mcp gene of all the ascoviruses examined and CIV can form a stem-loop structure, and the ends of the mcp gene transcripts of SeAV-5a are within the predicted stem-loop region. This suggests that the stem-loop structure of the mcp gene might be involved in transcription termination.

Sequence and organization of the Trichoplusia ni ascovirus 2c (Ascoviridae) genome

The complete Trichoplusia ni ascovirus 2c (TnAV-2c) genome sequence was determined. The circular genome contains 174,059 bp with 165 open reading frames (ORFs) of greater than 180 bp and two major homologous regions (hrs). The genome is quite A+T rich at 64.6%. Fifty-four ORFs had homologues in other insect viruses, such as ascoviruses, iridoviruses, baculoviruses and entomopoxviruses; 30 ORFs showed low identities with those from different parasitic protozoa and 12 ORFs were unique to TnAV-2c. TnAV-2c has 15 ORFs that could be grouped into six gene families. Three major conserved repeating sequences were identified and were interspersed in two regions. BLAST analyses revealed that there were 16 enzymes involved in gene transcription, DNA replication, and nucleotide metabolism. TnAV-2c has 12 and 25 ORFs sharing high identities with ascovirus and iridovirus homologues, respectively. The codon usage bias appears to be more similar to Spodoptera frugiperda ascovirus 1a than to iridoviruses.

Characterization of three ascovirus isolates from cotton insects

Three new ascovirus isolates were discovered from lepidopteran larvae in cotton fields in Blackville, South Carolina, USA, and were named TnAV-2c, TnAV-2d, and HvAV-3f. TnAV-2c and TnAV-2d were compared by restriction endonuclease (REN) profiles and found to be similar. HvAV-3f was isolated from Helicoverpa zea, and bears remarkable dissimilarity in REN profiles to the reported SeAV-5a from Spodoptera exigua but DNA hybridization shows they are closely related. Major capsid protein (MCP) and delta DNA polymerase from the three isolates were sequenced, which suggests the three isolates are novel. Phylogenetic analyses showed that TnAV-2c is distantly related to other lepidopteran ascoviruses. HvAV-3f and SeAV-5a may also be variants of the same species based on Southern, Western, and MCP/DNA polymerase gene sequence analyses. High levels of TnAV-2 infection in an H. zea population (as high as 74%) were recorded in a cotton field in Blackville, SC. Observations in this field showed that infection by ascovirus altered the feeding behavior of H. zea larvae.