Structural characterization of the proteins encoded by adenovirus early region 2A

Characterization of the cryptic AV3 promoter of ageratum yellow vein virus in prokaryotic and eukaryotic systems

A cryptic prokaryotic promoter, designated AV3 promoter, has been previously identified in certain begomovirus genus, including ageratum yellow vein virus isolate NT (AYVV-NT). In this study, we demonstrated that the core nucleotides in the putative -10 and -35 boxes are necessary but not sufficient for promoter activity in Escherichia coli, and showed that AYVV-NT AV3 promoter could specifically interact with single-stranded DNA-binding protein and sigma 70 of E. coli involved in transcription. Several AYVV-NT-encoded proteins were found to increase the activity of AV3 promoter. The transcription start sites downstream to AV3 promoter were mapped to nucleotide positions 803 or 805 in E. coli, and 856 in Nicotiana benthamiana. The eukaryotic activity of AV3 promoter and the translatability of a short downstream open reading frame were further confirmed by using a green fluorescent protein reporter construct in yeast (Saccharomyces cerevisiae) cells. These results suggested that AV3 promoter might be a remnant of evolution that retained cryptic activity at present.

Use of Xenopus oocytes to study the expression of cloned genes and translation of mRNA

One of the most active areas of research in the field of molecular biology is the examination of the mechanisms associated with the regulation of gene expression. Our understanding of the events in eukaryotic transcription has been aided by the ability to test the expression of various genomic DNA constructs after their microinjection into the germinal vesicle of Xenopus oocytes. This in vivo transcription system has permitted the analysis of the involvement of cis-acting DNA sequences and the examination of the effect of co-injected trans-acting factors on gene expression. Furthermore, the Xenopus oocyte has been employed widely as an in vivo translation system. Not only is the Xenopus oocyte system a sensitive assay for the translation of rare mRNAs but it also has the ability to post-translationally modify and compartmentalize numerous types of proteins. Finally, the Xenopus oocyte has proven useful in the procedures associated with the cloning and screening of cDNAs.

The binding of in vitro synthesized adenovirus DNA binding protein to single-stranded DNA is stimulated by zinc ions

We have synthesized wild type DNA binding protein (DBP) of adenovirus type 5 (Ad5) and several truncated forms of this protein by a combination of in vitro transcription and translation. The proteins obtained were tested for binding to a single-stranded DNA-cellulose column. It could be shown that for binding of in vitro synthesized wild type DBP, it is necessary to add zinc ions to the in vitro translation system. Binding studies with the truncated proteins revealed that deletion of the carboxyl-terminal 46 amino acids abolishes DNA binding.

Isolation and analysis of adenovirus type 5 mutants containing deletions in the gene encoding the DNA-binding protein

A genetic system is described which allows the isolation and propagation of adenovirus mutants containing lesions in early region 2A (E2A), the gene encoding the multifunctional adenovirus DNA-binding protein (DBP). A cloned E2A gene was first mutagenized in vitro and then was introduced into the viral genome by in vivo recombination. The E2A mutants were propagated by growth in human cell lines which express an integrated copy of the DBP gene under the control of a dexamethasone-inducible promoter (D. F. Klessig, D. E. Brough, and V. Cleghon, Mol. Cell. Biol. 4:1354-1362, 1984). The protocol was used to construct five adenovirus mutants, Ad5d1801 through Ad5d1805, which contained deletions in E2A. One of the mutants, Ad5d1802, made no detectable DBP and thus represents the first DBP-negative adenovirus mutant, while the four other mutants made truncated DBP-related polypeptides. All five mutants were completely defective for growth and plaque formation on HeLa cell monolayers. Furthermore, the two mutants which were tested, Ad5d1801 and Ad5d1802, did not replicate their DNA in HeLa cells. The mutant Ad5d1804 encoded a truncated DBP-related protein which contained an entire amino-terminal domain derived from the host range mutant Ad5hr404, a variant of Ad5 which multiplies efficiently in monkey cells. While results of a previous study suggest that the amino-terminal domain of DBP could act independently of the carboxyl-terminal domain to enhance late gene expression in monkey cells, the Ad5d1804 polypeptide failed to relieve the block to late viral protein synthesis in monkey cells. The mutant Ad5d1802 was used to study the role of DBP in the regulation of early adenovirus gene expression in infected HeLa cells. These experiments show that E2A mRNA levels are consistently reduced approximately fivefold in Ad5d1802-infected cells, suggesting either a role for DBP in the expression of its own gene or a cis-acting defect caused by the E2A deletion. DBP does not appear to play a significant role in the regulation of adenovirus early regions 1A, 1B, 3, or 4 mRNA levels in infected HeLa cell monolayers since wild-type Ad5- and Ad5d1802-infected cells showed very little difference in the patterns of expression of these genes.

Analysis of expression of adenovirus DNA (fragments) by microinjection in Xenopus oocytes. Independent synthesis of minor early region 2 proteins

Injection of whole adenovirus DNA into Xenopus oocytes results in the synthesis of large amounts of the early region 2A DNA-binding protein (E2A-DBP) and smaller amounts of polypeptide IX. The lack of synthesis of any functional messenger RNAs transcribed from the major late promotor at 16.3 map units is remarkable. Cleavage of the adenovirus DNA outside the E2A gene proper by restriction enzymes decreases synthesis of the DBP to about 10% of the amount produced after injection of intact DNA. On the other hand, presence of the terminal (Bellett) protein on the injected template enhances DBP synthesis considerably. Experiments with injected DNA restriction fragments, as well as reconstructed genes cloned into pBR322, indicate that efficient synthesis of DBP in oocytes requires the presence of either or both of the two main promoters from which the E2A gene is transcribed plus an intact 3' end of the gene. In the absence of any known promotor, 100-fold lower amounts of otherwise normal DBP are produced. Unlike in a regular infection, synthesis of DBP in oocytes does not require the product of the E1A gene. The same series of experiments also demonstrates that the DBP, a phosphoprotein, is the substrate of a cellular rather than a virus-encoded protein kinase. Two minor E2A proteins, although colinear with the major DBP, are synthesized independently. Synthesis of a 44,000 Mr protein, probably corresponding to the carboxy-terminal 360 amino acid residues of the DBP, is not decreased after injection of "promotorless" E2A genes. Unlike the 44,000 Mr protein, production of a 67,000 Mr protein (carboxy-terminal 483 amino acid residues) by one DNA-construct is probably directed by a T-A-T-A-A-A-T-A sequence in the vector DNA.