Variants of open reading frame Bm126 in wild-type Bombyx mori nucleopolyhedrovirus isolates exhibit functional differences

Abstract

The open reading frame (ORF) 126 (Bm126) of Bombyx mori nucleopolyhedrovirus (BmNPV) is a homologue of Ac150 and belongs to the baculovirus 11K protein family. Bm126 was amplified from BmNPVs isolated from five different regions of China. Sequence analysis showed that the isolates had two different subtypes of Bm126, Bm126-SX and Bm126-GD, and both were different from that of the BmNPV T3 isolate. All of the BM126 ORFs contained a hydrophobic N terminus and a C6 motif at their C terminus, but the sequence between the N terminus and C6 motif varied in each isolate. The function of Bm126 was studied using bacmid BmBacJS13 derived from a BmNPV containing Bm126-SX. A 39 rapid amplification of cDNA ends showed that the transcript of Bm126 was first detected at 6 h post-infection. A Bm126-knockout bacmid was constructed in which the majority of the coding region of Bm126 was deleted. Subsequently, the gene was repaired with Bm126-SX or Bm126-GD and tested for infectivity. The deletion of Bm126 had no obvious effect on the budded virus growth curve and the mean lethal dose of the occlusion bodies (OBs); however, the mean survival time of the larvae infected with Bm126-null virus was significantly delayed compared with that of the control virus. The delay was rescued by repairing the deletion with Bm126-SX but not with Bm126-GD. In addition, the virus repaired with Bm126-GD showed a significant increase in OB yield, both in vitro and in vivo.

Interaction of the baculovirus occlusion-derived virus envelope proteins ODV-E56 and ODV-E66 with the midgut brush border microvilli of the tobacco budworm, Heliothis virescens (Fabricius)

“…Deletion mutants of two of the Ac150 subtypes exhibited an increase in survival time, but not a decrease in oral infectivity in agreement with the first study (Hao et al, 2009).…”

“…Both scenarios have been documented in the literature (Cory and Myers, 2003), and are a key element in the virulence of baculovirus populations in the field. There is also evidence to suggest that mixtures of wildtype and pif knockout polyhedra produce a more virulent population than the wild type alone (Hao et al, 2009 -pif-0, pif-1, pif-2, and pif-3 found that mixtures of these viruses were also noninfectious to the host per os . This result suggests that virus infection requires all four of these PIFs present on the same ODV, and is not unexpected given their conservation across all baculoviruses.…”

“…In addition, the three proteomic analyses have identified a plethora of virus specific proteins, which may be included in the OB or ODV and may be essential to oral infection of specific hosts . Current studies of the PIFs already indicate that they have different functions in different host insects, or different homologs have different functions within the same host (Hao et al, 2009;Zhang et al, 2005).…”

“…In a natural infection the insects would ingest polyhedra, not liberated ODV, and would be exposed to the enhancing effects observed with ingestion of the polyhedra. A recent study of BmNPV field isolates found multiple subtypes of the Ac150 homolog, all of which had the N-terminal and C-terminal domains, but varied with respect to the presence of an interior RGD motif (Hao et al, 2009).…”

“…Among all 52 lepidopteran core genes, 14 are ODV envelope proteins or found within the ODV (excluding polyhedrin), while 9 of the 30 core baculovirus genes encode proteins that are present in the ODV. Notably, a class of proteins has been described, the per os infectivity factors (PIFs), which include five ODV envelope proteins that play a role in oral infectivity: P74-PIF-0 (ac138), PIF-1 (ac119), PIF-2 (ac22), PIF-3 (ac115), and PIF-4 (ac96) Haas-Stapleton et al, 2004;Hao et al, 2009;Kikhno et al, 2002;Yao et al, 2004). The first four PIFs are conserved across the Baculoviridae, and PIF-4 is conserved across the lepidopteran baculoviruses.…”