The mechanism for down-regulation of E1a expression by products encoded in the E3 transcription unit of human adenovirus types 2 and 5, that occurs in infected L929 cells, has been investigated further. We show that the phenomenon occurs in different mouse cells and also in some human cells suggesting that the observations have relevance to natural human infections. We also provide evidence that probably all viral proteins are down-regulated by E3 products, although to different extents, but that host proteins are unaffected. Whereas E1a protein levels and synthesis are reduced in the presence of E3 products, Ela protein half-life and polysomal E1a RNA levels and size distribution are not. These data suggest that E3 products down-regulate E1a protein levels by interfering with the translation of E1a-specific mRNA. Studies were additionally carried out with mutant adenoviruses containing different defects in the E3 transcription unit. Based on these studies it seems likely that the E3 14·5K and 10·4K proteins are crucially involved in E1a down-regulation. Our data are discussed in terms of strategies for immune evasion by group C human adenoviruses.


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