Alternate translation initiation on hepatitis B virus X mRNA produces multiple polypeptides that differentially transactivate class II and III promoters.

The hepatitis B virus X gene encodes a transcription activator which stimulates the synthesis of RNAs from a variety of class II and III promoter elements. In this report, we present a mutational analysis which genetically demonstrates that the X gene actually encodes two, and possibly three, related polypeptides from a single mRNA using alternate translation initiation from any of three in-frame AUG codons. Genetic analysis shows that translation initiates at the 5' proximal AUG of X mRNA and produces a full-length 17-kDa X protein but in addition also likely initiates at either of two conserved, in-frame AUG codons, producing two amino-terminally truncated X proteins presumably of 8 and 6.6 kDa. Expression of mRNAs capable of encoding only one of each X protein all individually transactivate class III (RNA polymerase III)-transcribed promoters. However, class II (RNA polymerase II)-transcribed promoters displayed various requirements for the different X proteins. Expression of two X proteins, the 17- and 6.6-kDa species, was required to activate transcription of the simian virus 40 enhancer/early promoter. In contrast, activation of an NF-kappa B-dependent promoter was carried out only by mRNAs encoding the full-length 17-kDa X protein. These results indicate that the X gene encodes several related proteins that possess different transcriptional regulatory activities.