Sequence arrangement and protein coding capacity of the adenovirus type 2 "i" leader

The adenovirus type 2 (Ad-2) "i" leader is an RNA segment which is preserved in some mRNA species from the Ad-2 late transcription unit. It maps between the second and third segments of the standard tripartite leader. We located the boundaries of the i leader in genomic Ad-2 DNA and determined its nucleotide sequence. The leader contains an ATG initiator near its 5' boundary, followed by a reading frame which is open for translation. We suggest that the i leader constitutes an Ad-2 coding sequence whose novel position within the leader of major late transcription unit messengers allows it to be translated in preference to coding sequences in mRNA main bodies. The i leader potentially contributes to the coding sequences of a family of proteins. Also, a Northern blot analysis of late mRNAs containing the i leader suggests that it may be retained in the leaders of many different late transcription unit mRNAs. We compare the i leader to the simian virus 40 agnogene.