The matrix region is responsible for the differential ability of two retroviruses to function as helpers for vector propagation.

We have investigated the ability of two related reticuloendotheliosis viruses to propagate a spleen necrosis virus (SNV) based retroviral vector in canine osticosarcoma (D17) cells. Reticuloendotheliosis virus strain A (REV-A) consistently propagated the vector more efficiently than SNV in cell culture. To identify the area of the viral genome responsible for the superior helper function of REV-A, we constructed chimeric viruses between SNV and REV-A. Analysis of helper function indicated that a virus comprised of the SNV genome, but containing the matrix region of REV-A, could propagate the vector as well as REV-A. Although REV-A is also a superior virus for vector propagation in chicken embryo fibroblast cells, the region of the viral genome that confers superior helper function does not map to the gag region of REV-A in this cell type.