Human cytomegalovirus UL69 protein is required for efficient accumulation of infected cells in the G(1) phase of the cell cycle

Human cytomegalovirus blocks cell-cycle progression in the G(1) compartment upon infection of primary human fibroblasts, The virus-coded UL69 protein can institute a G(1) block when expressed in cells in the absence of virus infection. We have constructed a cytomegalovirus mutant, TNsubUL69, that la cks the UL69 coding region. This virus grows slowly in fibroblasts, but pro duces a wild-type yield after an extended delay. It grows with normal kinet ics in cells coinfected with a recombinant retrovirus, retroUL69, which exp resses UL69 protein, demonstrating that its growth defect results from the mutation in the UL69 gene. UL69 protein is packaged within virus particles, and it was possible for us to produce two types of virus stocks. TNsubUL69 (+pUL69) lacks the UL69 gene but contains UL69 protein in virus particles. It is produced by growth in fibroblasts that are coinfected with retroUL69, TNsubUL69(-pUL69) lacks the UL69 gene and protein. It is produced by growt h in fibroblasts that do not contain UL69 protein. The mutant virions lacki ng both the UL69 gene and protein fail to induce a cell-cycle block with no rmal efficiency, whereas the mutant particles lacking the gene but containi ng the protein can institute the block. These results are consistent with t he view that the UL69 protein contributes to the cytomegalovirus-induced ce ll-cycle block, and they suggest that UL69 protein delivered to cells withi n virions can induce the block without the synthesis of additional UL69 pro tein encoded by the infecting viral genome.