INDUCIBLE GENE-EXPRESSION FROM AFRICAN SWINE FEVER VIRUS RECOMBINANTS- ANALYSIS OF THE MAJOR CAPSID PROTEIN P72

A method to study the function of individual African swine fever virus (ASFV) gene products utilizing the Escherichia coli lac repressor-ope rator system has been developed. Recombinant viruses containing both t he lacI gene encoding the lac repressor and a strong virus late promot er modified by the insertion of one or two copies of the lac operator sequence at various positions were constructed. The ability of each mo dified promoter to regulate expression of the firefly luciferase gene was assayed in the presence and in the absence of the inducer isopropy l beta-D-thiogalactoside (IPTG). Induction and repression of gene acti vity were dependent on the position(s) of the operator(s) with respect to the promoter and on the number of operators inserted. The ability of this system to regulate the expression of ASFV genes was analyzed b y constructing a recombinant virus inducibly expressing the major caps id protein p72. Electron microscopy analysis revealed that under nonpe rmissive conditions, electron-dense membrane-like structures accumulat ed in the viral factories and capsid formation was inhibited. Inductio n of p72 expression allowed the progressive building of the capsid on these structures, leading to assembly of ASFV particles. The results o f this report demonstrate that the transferred inducible expression sy stem is a powerful tool for analyzing the function of ASFV genes.