Transcriptional analyses of baculovirus polyhedrin and foreign gene expression relative to baculovirus p10 mRNA levels

Comparisons have been made between the p10 and polyhedrin mRNA levels recovered from Spodoptera frugiperda cells infected with Autographa californica nuclear polyhedrosis virus (AcNPV). In molar terms and from 18 h post-infection (p.i.), the polyhedrin mRNA species increased to levels one and a half times to twice as high as the p10 levels. The influence of the polyhedrin leader sequence on the expression of a foreign gene under the control of the polyhedrin promoter was investigated using a series of four recombinant baculoviruses expressing the lymphocytic choriomeningitis virus nucleocapsid (N) protein gene. The different recombinants varied in the length and composition of the upstream polyhedrin mRNA leader sequence. The recombinant containing the full-length polyhedrin leader sequence gave levels of N mRNA comparable to those of AcNPV polyhedrin mRNA. These levels were either equal to (12 h p.i.) or higher (18 to 42 h p.i.) than the p10 levels at corresponding times. Three other recombinants, with different lengths of leader sequence, accumulated significantly lower quantities of N mRNA in comparison to the p10 mRNA levels. However the mRNA levels for the three recombinants were similar (20 to 50% of the p10 level) and did not correspond to their N protein expression levels. By comparing the mRNA and protein levels, it is concluded that the sequence between -8 to +1 of the AcNPV polyhedrin translation-initiating ATG has an important function for mRNA transcription (or accumulation), while the sequences between -32 to -8 affect the overall translation efficiencies.