via the respiratory or oral route with only mild or no symptoms which consisted of mild upper 
respiratory symptoms. Importantly, oral administration of live adenovirus (serotypes 4 and 7) 
has been used successfully in large numbers of military recruits ( > 5 million individuals) as a 
vaccination against the natural respiratory infection which commonly occurs in this group 
(Chanock et.al., 1966; Couch et.al., 1963; Top et.al., 1971a; Top et.al., 1971b). No clinical 
sequela were noted in individuals in these studies. 
The molecular biology of adenoviruses (reviewed in Ginsberg, 1984; Horowitz, 1990a) has 
long been the subject of active study predominantly as a model for eukaryotic gene expression. 
The viral genome is organized into several early and late regions each of which plays a specific 
role within the life cycle of the viral infection (Figure 2). There are four early regions El, E2, 
E3, and E4, as well as one major late region with five different principal transcriptional units 
derived from alternative transcription and splicing of the region. In addition, there are several 
minor intermediate and/or late regions. The El region is of special interest for several reasons. 
First, it is active immediately, as soon as the viral genome enters the cell nucleus and due to 
utilization of cellular transcriptional control factors to activate this region. Second, the El 
region is the regulator of all of the other early regions and activation of the El region is 
required for their activation. Third, the El region, which is composed of the Ela and Elb 
genes (Figure 3) is also responsible for cellular transformation which can be demonstrated in 
cultured cells. The E2 region codes for proteins related to replication of the adenoviral genome 
(including DNA binding proteins and a viral DNA polymerase). The E3 region codes for 
polypeptides involved in viral mechanisms which interact with the host cell and effect immune 
recognition of the infected cell. This region can be safely deleted with no apparent effect on the 
quantity or kinetics of replication of an otherwise "wild type" adenovirus. The E4 region, 
together with the E2 region, serve to upregulate the major late transcriptional unit which is tran- 
scribed from the major late promoter (MLP). Transcripts from the MLP initiate with a tripartite 
leader (TPL) which increases the relative efficiency of translation of viral mRNA, a function 
which appears to be positively influenced by an intermediate to late RNA transcript from the 
VAj region. The major late region codes for almost all of the structural polypeptides including 
hexon, pen ton base, fiber, core DNA binding proteins and other minor proteins which form the 
capsid. 
Human adenoviruses undergo a replicative infection cycle in those cells which are permissive 
for viral DNA replication (human respiratory epithelial cells). The respiratory epithelium of the 
cotton rat (Sigmadon hispidus) is also permissive for human adenoviruses and thus, the cotton 
rat provides a good model of the human lung for evaluating human adenoviruses. In contrast 
to retrovirus, there is no obligate proviral phase and, in fact, adenoviruses rarely integrate into 
the host cell genetic material. Briefly, during an infection, the adenovirus first attaches to the 
cell membrane by specific cell-surface receptors and is quickly internalized via clatharin coated 
pits into endosomes. Acidification of the endosome leads to a conformational change within the 
viral capsid polypeptides which leads to escape of the virion through endosomolysis. The virion, 
now free in the cytoplasm is transported to the nuclear membrane mediated through nuclear 
targeting signals within the capsid polypeptides. The adenoviral genome is then transported or 
injected into the cell nucleus where the genetic program unfolds. The El region, being 
constitutively active, activates El, E2, and E3 which remain active. After about eight hours, 
DNA replication begins and subsequently, the major late unit is active leading to formation of 
approximately 10,000 virions per cell within the nucleus of the cell. Cell death occurs 
approximately 30 - 40 hours after initiation of infection releasing the intracellular virions. 
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Recombinant DNA Research, Volume 17 
