DNA sequence of the early E3 transcription unit of adenovirus 5

The DNA sequence of the early E3 transcription unit of adenovirus 5 (Ad5) has been determined and it has been compared to Ad2, as published previously [J. Hérissé, G. Courtois, and F. Galibert (1980), Nucl. Acids Res. 8, 2173-2192; J. Hérissé and F. Galibert (1981), Nucl. Acids Res. 9, 1229-1240]. The E3 regions of Ad5 and Ad2 are quite homologous despite being nonessential for Ad growth in cultured cells. The major differences are "gaps" that exist either in Ad5 or Ad2 in intergenic regions. The conservation of sequences suggests that E3 plays a beneficial role in natural infection of humans. E3 appears to encode about seven to nine proteins; based on sequence, seven of these may be membrane proteins. Thus, E3 may be a transcription unit devoted to the synthesis of membrane proteins. The E3 genes lie essentially one after the other along the genome, and which gene is expressed from a given primary transcript is determined by the choice of the 3' end site and the 5' and 3' splice sites. Almost all E3 mRNAs contain nonfunctional AUGs that are 5' to the initiation codon. Codon usage is nonrandom. Although the CG dinucleotide frequency is low, CG clusters exist in the promoter and other regions.