VA RNAs from avian and human adenoviruses: dramatic differences in length, sequence, and gene location.

Human adenoviruses encode low-molecular-weight RNAs, so-called VA RNAs, which are transcribed by RNA polymerase III. These RNAs are required for an efficient translation of viral mRNAs late after infection. The genes for the VA RNAs in the genome of CELO virus were mapped and characterized. The results showed a number of surprising differences between CELO virus and human adenovirus type 2 (Ad2). Thus, the CELO virus genome encoded only one VA RNA species, in contrast to human Ad2, which encoded two distinct species. The VA RNA from CELO virus was much shorter than the Ad2 VA RNAs (90 nucleotides compared with 160 nucleotides), and there existed no detectable primary sequence homology between them. The predicted secondary structure of CELO virus VA RNA was, however, similar to that of the Ad2 VA RNAs, implying that the folding rather than the primary sequence was the important feature for biological activity. CELO VA RNA also stimulated translation in a transient expression assay, as did the Ad2 counterparts, albeit with a much lower efficiency. The location of the gene for CELO VA RNA also differed from all previously characterized serotypes, suggesting that the genome organization of avian and human adenoviruses are different. Finally, termination of CELO VA RNA transcription occurred in a TTATT sequence which is unique as a stop signal for RNA polymerase III transcription.