Human cytomegalovirus blocks cell-cycle progression in the G1 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 lacks the UL69 coding region. This virus grows slowly in fibroblasts, but produces a wild-type yield after an extended delay. It grows with normal kinetics in cells coinfected with a recombinant retrovirus, retroUL69, which expresses 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 growth in fibroblasts that do not contain UL69 protein. The mutant virions lacking both the UL69 gene and protein fail to induce a cell-cycle block with normal efficiency, whereas the mutant particles lacking the gene but containing the protein can institute the block. These results are consistent with the view that the UL69 protein contributes to the cytomegalovirus-induced cellcycle block, and they suggest that UL69 protein delivered to cells within virions can induce the block without the synthesis of additional UL69 protein encoded by the infecting viral genome.H uman cytomegalovirus (HCMV), a ␤-herpesvirus, is a ubiquitous pathogen. Although HCMV infection generally is asymptomatic in healthy children and adults, it is the leading viral cause of birth defects (1) and a major cause of morbidity and mortality in immunocompromised individuals (2).HCMV infection blocks cell-cycle progression in primary human fibroblasts (3-6). The arrest occurs at multiple phases of the cell cycle, including a point late in the G 1 compartment. We have previously shown that expression of the HCMV UL69 protein in human tumor cells or primary fibroblasts blocks cell-cycle progression (7). Cells expressing UL69 protein after transfection with an expression plasmid or infection with a recombinant retrovirus accumulate in the G 1 compartment of the cell cycle. UL69 protein is a constituent of the virion (8) that exhibits transcriptional regulatory activity (9, 10). The mechanism by which UL69 institutes a cell-cycle block is not known.In this paper, we describe the construction and analysis of a UL69-deficient HCMV mutant, TNsubUL69. The mutant virus lacking the UL69 gene and protein is defective for replication in fibroblasts, producing a normal yield only after a significant delay. The mutant virus lacking the UL69 gene and protein fails to efficiently induce a G 1 block after infection of fibroblasts, whereas the mutant virus lacking the UL69 gene but containing UL69 protein can institute the block. These results demonstrate that UL69 protein contributes to the cell-cycle block obser...

The VHL tumor suppressor gene has previously been reported to encode a protein of 213 amino acid residues. Here we report the identi®cation of a second major VHL gene product with an apparent molecular weight of 18 kD, pVHL18, which appears to arise from alternate translation initiation at a second AUG codon (codon 54) within the VHL open reading frame. In vitro and in vivo studies indicate that the internal codon in the VHL mRNA is necessary and su cient for production of pVHL18. pVHL18 can bind to elongin B, elongin C, and Hs-CUL2. When reintroduced into renal carcinoma cells that lack a wild-type VHL allele, pVHL18 suppresses basal levels of VEGF expression, restores hypoxiainducibility of VEGF expression, and inhibits tumor formation in nude mice. These data strongly support the existence of two distinct VHL gene products in VHL tumor suppression.

The human cytomegalovirus IRS1 and TRS1 open reading frames encode immediate-early proteins with identical N-terminal domains and divergent C-terminal regions. Both proteins have been shown previously to activate reporter genes in transfection assays in cooperation with other viral gene products. We have constructed two viruses carrying substitution mutations within either the IRS1 or TRS1 open reading frame. ADsubIRS1 failed to produce the related IRS1 and IRS1 263 proteins, but it replicated with normal kinetics to produce a wild-type yield in human fibroblasts. The addition in trans of the IRS1 263 protein, which antagonizes the ability of IRS1 and TRS1 proteins to activate reporter genes, did not inhibit the growth of the mutant virus. ADsubTRS1 failed to produce the TRS1 protein, and it generated an ϳ200-fold-reduced yield of infectious virus in comparison to its wild-type parent. Viral DNA accumulated normally, as did a set of viral mRNAs that were monitored in ADsubTRS1-infected cells. However, two tegument proteins were partially mislocalized and infectious virus particles did not accumulate to normal levels within ADsubTRS1-infected cells. Further, infectious ADsubTRS1 particles sedimented abnormally in a glycerol-tartrate gradient, indicating that the structure of the mutant particles is aberrant. Our analysis of the ADsubTRS1 phenotype indicates that the TRS1 protein is required, either directly or indirectly, for efficient assembly of virus particles.

FlbD is a transcriptional regulatory protein that negatively autoregulates fliF, and it is required for expression of other Caulobacter crescentus flagellar genes, including flaN and flbG. In Mullin, J. Bacteriol. 175:367-376, 1993). Three DNA fragments, each carrying anftr4 mutation that resulted in elevatedfliF transcript levels in vivo, were defective in binding to FlbDc87 in vitro. We also found that a missense mutation in the recognition helix of the putative helix-turn-helix DNA-binding motif of FlbDc87 resulted in defective binding toftr4 in vitro. These data suggest that the binding of FlbDc87 to ftr4 is relevant to negative transcriptional regulation offliF and that FlbD functions directly as a repressor. Footprint analysis showed that FlbDc87 also makes close contacts with specific nucleotides inftr, tr2, andftr3 in thefjaN-flbG promoter region, and some of these nucleotides were shown previously to be required for regulated transcription offlaN andflbG (D. A. Mullin and A. Newton, J. Bacteriol. 175:2067-2076. Footprint analysis also revealed a newftr-like sequence,ftr5, at -136 from the transcription start site offlbG. Our results demonstrate that FlbD contains a sequence-specific DNA-binding activity within the 87 amino acids at its carboxy terminus, and the results suggest that FlbD exerts its effect as a positive and negative regulator of C. crescentus flagellar genes by binding to ftr sequences.Caulobacter crescentus has a unique cell division cycle that yields two morphologically distinct cell types, and it has provided a tractable and fruitful model for investigating interrelated aspects of temporal and spatial regulation that result in cell differentiation (for reviews, see references 4, 39, and 53). Most of these studies with C. crescentus have concentrated on morphogenesis of the flagellum, and they have been greatly facilitated by the availability of large numbers of nonmotile mutants (23).Approximately 50 genes are required for assembly and function of the C. crescentus flagellum (16), and epistasis studies have revealed that these flagellar genes are arranged in a regulatory hierarchy consisting of four levels or classes of genes ( Fig. 1) (6,40,44,58,62). The flagellin genes flgK and flgL occupy level IV at the bottom of the hierarchy, and their expression depends on the level III genes (30, 44) which depend, in turn, on level II genes for their expression (9,10,35,40). Although trans-acting genetic requirements specific for expression of the level II genes have been searched for by several laboratories, none has been reported. If found, such a gene or genes would occupy level I.In addition to being under hierarchical control, these flagellar genes are also under cell cycle control because their * Corresponding author. Phone: (504) 865-5546. Fax: (504) 865- 6785.expression is restricted to a discrete interval in the cell cycle, and it has been suggested that a step in the DNA replication pathway plays a role in timing the expression of flagellar genes (12,46,54). Studies to understand...