Transfer vectors for maximal expression of passenger genes in the Bombyx mori nuclear polyhedrosis virus expression system.

A series of Bombyx mori nuclear polyhedrosis virus (Bm-NPV) transfer vectors has been developed containing various lengths of the polyhedrin promoter, including sequences 3' of the initiation codon. The ATG initiation codon was mutated in some of these vectors to allow for the production of authentic nonfusion proteins. The ability of the various polyhedrin promoter constructs to direct expression of foreign gene sequences was assessed using two test genes, chloramphenicol acetyl transferase (cat), and human metallothionein II. Accumulation of cat mRNA and nonfused protein was low when only polyhedrin promoter sequences to -8 (relative to the translational start site of polyhedrin mRNA) were included in the transfer vector, but cat expression was comparable with that of the wild-type polyhedrin gene when promoter sequences to +5 were present. Further addition of polyhedrin gene sequences to +26 or +94 resulted in no further increase in expression. Similar results were obtained for expression of human metallothionein II, where constructs encoding polyhedrin-metallothionein fusion proteins containing polyhedrin sequences to at least +5 resulted in high levels of mRNA and protein accumulation. The expression vectors containing the +5, +26, or +94 BmNPV polyhedrin promoter can thus be used to direct maximal levels of production of nonfused proteins (when the polyedrin ATG has been mutated) or of fusion proteins, depending on which is more suitable for a particular application. These new vectors are a useful addition to those presently available and should increase the utility of the BmNPV expression system for large-scale protein production.