The messenger RNAs from the transforming region of human adenoviruses

The E1B transcription map of the enteric adenovirus type 41

Enteric adenovirus type 41 (Ad41) is defective for growth in conventional established cell lines. Ad41 is dependent on the Ad5 early regions E1A/E1B since it cannot grow in HEK cells but only in 293 HEK cells transformed by Ad5 E1 region. However, Hep-2 cells have also been shown to support the growth of Ad41 to some extent. The nucleotide sequence of the E1B region of the Ad41 strain D389 has been determined. When compared to the corresponding region of the Ad41 prototype strain (Tak) the degree of homology in the DNA sequences was close to 100%. The mRNAs from the E1B region of the Ad41 strain D389 have been studied by Northern blot, primer extension, and polymerase chain reaction-cDNA analysis. E1B transcripts corresponding to Ad2 14 S, 22 S, and 9 S mRNAs were identified but no 13 S mRNA equivalent was detected, a pattern similar to that seen in the Ad40 and Ad12 transcription maps. However, the Ad41 E1B 14S mRNA equivalent has one additional small exon of 23 nucleotides, created by a donor and an acceptor splice site located at positions not seen in other E1B transcripts of human adenoviruses analyzed so far. The coding potential for E1B 19K, 55K, and 15K proteins and for pIX is retained in the Ad41 transcripts. In contrast to other adenoviruses, except for the closely related Ad40, the ORF of pIX starts in the intron of the 22 S mRNA.

Expression and interactions of human adenovirus oncoproteins

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Physical organization of the enteric adenovirus type 41 early region 1A

Enteric adenovirus types 40 and 41 (Ad40 and Ad41), representing subgenus F, differ from all other human adenoviruses by being so fastidious that productive replication does not occur in conventional established cell lines. They are dependent of the Ad5 early regions E1A and E1B since they can not grow in HEK cells, only in 293 HEK cells transformed by Ad5 E1. The overall genetic organization of Ad41 E1A is similar to the E1A region of other characterized human adenoviruses but it is slightly shorter, comprising 1350 bp. The inverted terminal repeat (ITR) at the 5' end of both Ad40 and AD41 consists of 163 nucleotides, being similar to the ITR of Ad12 (subgenus A) and longer than the ITRs of adenoviruses of subgenera B, C, and E. The early mRNA products (12 and 13 S) can be translated into a 222-amino acid (aa) and a 251-aa tentative protein, respectively. In a comparison of the Ad41 251-aa protein with corresponding peptides of Ad12, Ad7, Ad5, and Ad4, three conserved amino acid sequences CS1-CS3 can be found. In the second conserved domain CS2, which is particularly acidic, the homology is very high within all five serotypes compared. Only one among eight conserved amino acids differs in the Ad41 251-aa protein. Within CS1 and CS3 which exhibit a hydrophilic and a hydrophobic character, respectively, the amino acid composition of the Ad41 protein is less conserved than the corresponding regions in all other analyzed adenovirus types. Ten of 16 conserved amino acids in CS1 are shared by Ad41 and 18 of 23 conserved amino acids in CS3 are shared by Ad41.

Host range mutants of adenovirus type 12 E1 defective for lytic infection, transformation, and oncogenicity

The human adenovirus type 12 (H12) E1A region encodes two early proteins of 266 amino acid residues (266R) and 235R whilst the H12 E1B promoter directs the synthesis of two major proteins of 163R and 482R. To determine the functions of E1A and E1B in lytic infection and oncogenic transformation we have isolated and characterized a series of H12 E1 mutants. Mutant H12 hr 700 contains a point mutation in exon 1 that alters a single amino acid common to both the 266 and 235R proteins. This mutant synthesized reduced levels of E1 and structural proteins at delayed times in HEK cells, transformed BRK cells, and induced tumors in newborn rats at reduced efficiency compared to wild-type virus. The mutation in H12 in 600 truncates the 266R protein in its unique sequences but this mutant synthesized the 235R, E1B, and structural proteins at delayed times in HEK cells. H12 in 600 was nontransforming but induced rare tumors in newborn rats. A third E1A mutant H12 in 601 synthesized no E1A proteins, reduced levels of E1B and structural proteins at delayed times in lytic infections, and was not a transforming or oncogenic virus. Three E1B mutants were studied in detail. Both H12 hr 703 and H12 in 602 encode N-terminal truncated 482R proteins whereas H12 del 620 encodes an in-frame internally deleted 482R protein. All three synthesized reduced amounts of E1A proteins and the E1B 163R protein, identifying a regulatory function for the 482R protein. None of the E1B mutants could transform and only H12 del 620 could induce rare tumors in newborn rats. These results show that H12 oncogenesis requires the coordinated expression of the E1 proteins.

Adenovirus early region 1A protein increases the number of template molecules transcribed in cell-free extracts

Protein encoded by adenovirus early region 1A (E1A) stimulates transcription from adenovirus promoters in vivo. Here we show that this effect can be observed in vitro. In a run-off transcription assay from the adenovirus serotype 2 (Ad2) major late promoter, extracts prepared 20 hr postinfection were 5-15 times more active than mock-infected-cell extracts prepared in parallel. Similar results were observed for in vitro transcription from the protein IX and E3 adenovirus promoters, whereas a 2-fold increase was observed for the human beta-globin promoter. The increased activities of infected-cell extracts did not depend on the expression of viral late proteins or the small E1A-encoded proteins but did require expression of the large E1A protein. These results are consistent with the large E1A protein stimulating transcription in vitro as it does in vivo. By limiting in vitro transcription to one initiation per template, we found that the higher activity of an infected-cell extract was due to an increase in the number of templates transcribed. These results suggest that the large E1A protein either increases the number of active transcription factors in infected cells or facilitates the interaction of cellular transcription factors with promoter DNA.

Autoregulation of adenovirus E1A gene expression

We examined E1A gene expression by two evolutionarily divergent human adenoviruses, type 5 (subgroup C) and type 3 (subgroup B). Adenovirus type 3 (Ad3)-infected A549 cells contained much larger amounts of E1A-specific RNA than adenovirus type 5 (Ad5)-infected cells, from very early (3 h) through the late stages (20 h) after infection. The appearance of such abundant Ad3 E1A transcripts was delayed after infection of Ad5 E1A-expressing 293 cells, suggesting a down regulation of the Ad3 E1A gene by Ad5 E1A gene products. In a reciprocal manner, coinfection of A549 cells led to typically early and intense Ad3 E1A transcription and strongly inhibited transcription of the Ad5 E1A gene. Transient expression assays were developed so that the autoregulation of the E1A gene could be studied apart from the more complex background of infected cells. The DNA sequence surrounding the transcription start site of the Ad3 E1A gene was placed 5' to the sequence which encodes the bacterial chloramphenicol acetyltransferase gene. Cotransfection of HeLa cells with Ad3 or Ad5 E1A-expression plasmids increased the expression of the Ad3 E1A promoter-driven chloramphenicol acetyltransferase gene. Taken together, these results suggest dual autoregulatory features of adenovirus E1A gene expression. The positive and negative effects appear to be temporally distinguished under different conditions, both in viral infection and in transient assays with plasmid-cloned genes.