Abnormal expression of a late gene family L1 protein in monkey cells abortively infected with adenovirus type 2

Construction, characterization, and utilization of cell lines which inducibly express the adenovirus DNA-binding protein

To further our understanding of structure-function relationships within the multifunctional adenovirus DNA binding protein (DBP) a more diverse collection of mutants is necessary. DBP-expressing cell lines (gmDBP) were previously constructed that complemented DBP-negative mutants for viral growth. However, they did not allow severely defective viruses to form plaques. Since efficient mutant construction is reliant on plaque isolation of the desired mutant virus as a final step, additional gmDBP cell lines were constructed which allow all DBP-negative mutants to form plaques. Here we describe the construction and characterization of 12 new gmDBP cell lines. The utility of these lines was demonstrated by the efficient construction of a new defective mutant, H5in804, using a combination of DBP-expressing lines. The H5in804 mutation adds 22 amino acids at the carboxyl end of an otherwise wild type protein. Characterization of H5in804 revealed that it was altered in its ability to replicate viral DNA. The depression of DNA synthesis most probably results from a reduced ability of H5in804 DBP to bind ssDNA.

cis-acting elements and a trans-acting factor affecting alternative splicing of adenovirus L1 transcripts

Expression of the L1 region of adenovirus is temporally regulated by alternative splicing to yield two major RNAs encoding the 52- to 55-kilodalton (52-55K) and IIIa polypeptides. The distal acceptor site (IIIa) is utilized only during the late phase of infection, whereas the proximal site (52-55K) is used at both early and late times. Several parameters that might affect this alternative splicing were tested by using expression vectors carrying the L1 region or mutated versions of it. In the absence of a virus-encoded or -induced factor(s), only the 52-55K acceptor was used. Decreasing the distance between the donor and the IIIa acceptor had no effect. Removal of the 52-55K acceptor induced IIIa splicing slightly, implying competition between the two acceptors. Fusion of the IIIa exon to the 52-55K intron greatly enhanced splicing of the IIIa junction, suggesting that the IIIa exon does not contain sequences that inhibit splicing. Thus, the lack of splicing to the IIIa acceptor in the absence of a virus-encoded or -induced factor(s) is probably due to the absence of a favorable sequence and/or the presence of a negative element 5' of the IIIa splice junction, or both. The presence of several adenovirus gene products, including VA RNAs, the E2A DNA-binding protein, and the products of E1A and E1B genes, did not facilitate use of the IIIa acceptor. In contrast, the simian virus 40 early proteins, probably large T antigen, induced IIIa splicing. This result, together with those of earlier studies, suggest that T antigen plays a role in modulation of alternative RNA splicing.

Increased permissivity of monkey cells to human adenovirus multiplication is affected by culturing conditions and correlates with both synthesis of virion fiber protein and altered splicing of its mRNA

Human adenoviruses (Ad2) grow poorly in CV-1 (monkey) cells, resulting in an abortive infection. During abortive infections synthesis of the fiber protein is approximately 100-fold depressed compared to its synthesis in CV-1 cells productively infected with a host range mutant of adenovirus (Ad2hr400), while the corresponding mRNA is reduced only 10-fold. We found that passage of CV-1 cells under modified culture conditions resulted in loss of restrictiveness for Ad2 growth. As cells became more permissive, fiber polypeptide synthesis was enhanced although fiber mRNA levels did not necessarily increase. Analysis of the 5' ends of fiber message isolated from cells in various stages of permissivity showed a direct correlation between efficient synthesis of the fiber protein and an altered splicing pattern of the fiber message.

Characterization of adenovirus type 5 insertion and deletion mutants encoding altered DNA binding proteins

We have introduced insertion and deletion mutations in the cloned DNA binding protein (DBP) gene of adenovirus type 5. The mutated DBP genes were subsequently introduced in the viral genome by a combination of in vitro and in vivo methods. The resulting mutant viruses were tested for their viability in human 293 cells and an initial characterization of these viruses was performed. Viable mutants with insertions in the carboxyl-terminal portion of the gene could not be obtained. In contrast, a number of viable mutants were constructed that contained insertions or deletions in the amino-terminal half of DBP. Several of these, which covered the region between amino acid (aa) residues 39 and 81, were phenotypically wild type, implying that this segment is completely dispensable for DBP function. However, mutations altering the region encompassed by aa 2-38 were, at least, partially defective suggesting that this region is important for full activity of the protein.

cis-acting elements and a trans-acting factor affecting alternative splicing of adenovirus L1 transcripts

Expression of the L1 region of adenovirus is temporally regulated by alternative splicing to yield two major RNAs encoding the 52- to 55-kilodalton (52-55K) and IIIa polypeptides. The distal acceptor site (IIIa) is utilized only during the late phase of infection, whereas the proximal site (52-55K) is used at both early and late times. Several parameters that might affect this alternative splicing were tested by using expression vectors carrying the L1 region or mutated versions of it. In the absence of a virus-encoded or -induced factor(s), only the 52-55K acceptor was used. Decreasing the distance between the donor and the IIIa acceptor had no effect. Removal of the 52-55K acceptor induced IIIa splicing slightly, implying competition between the two acceptors. Fusion of the IIIa exon to the 52-55K intron greatly enhanced splicing of the IIIa junction, suggesting that the IIIa exon does not contain sequences that inhibit splicing. Thus, the lack of splicing to the IIIa acceptor in the absence of a virus-encoded or -induced factor(s) is probably due to the absence of a favorable sequence and/or the presence of a negative element 5' of the IIIa splice junction, or both. The presence of several adenovirus gene products, including VA RNAs, the E2A DNA-binding protein, and the products of E1A and E1B genes, did not facilitate use of the IIIa acceptor. In contrast, the simian virus 40 early proteins, probably large T antigen, induced IIIa splicing. This result, together with those of earlier studies, suggest that T antigen plays a role in modulation of alternative RNA splicing.

Characterization of the translational defect to fiber synthesis in monkey cells abortively infected with human adenovirus: role of ancillary leaders

Synthesis of the fiber protein of human adenovirus serotype 2 (Ad2) is 100-fold lower in abortively infected monkey cells, compared with productive infections, despite only a 5- to 10-fold reduction in fiber mRNA levels. Previously Anderson and Klessig (Proc. Natl. Acad Sci. USA 81:4023-4027, 1984) demonstrated a direct correlation between the productive nature of the infection, efficient synthesis of the fiber protein in vivo, and the presence of the x or y ancillary leaders on 10 to 25% of fiber messages. To determine at what level in translation these leaders might be important, the relative rate of initiation and elongation of each class of fiber message was assessed. The presence of the y ancillary leader in productively infected cells increased the rate of initiation about twofold, although translational elongation was similar on all fiber messages. However, the rate of elongation of all fiber messages was threefold slower in abortively infected than in productively infected cells. This reduced elongation rate in abortive infections was specific for fiber. The similar distribution of fiber mRNAs on polysomes in both infections suggests that initiation must also be partially blocked in abortive infections. Since the majority of the fiber mRNA even in productive infections did not contain the ancillary leaders, the initiation and elongation defects in the abortive infection cannot be fully explained by the absence of these leaders. Therefore, other factors in the infected cell must be influencing the rate of translation.