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.
Kwee
L
L
Department of Biochemistry, New York University Medical Center, New York 10016.
Lucito
R
R
Aufiero
B
B
Schneider
R J
RJ
D003062:Codon; D004742:Enhancer Elements, Genetic; D006515:Hepatitis B virus / Q000235:genetics*; D016328:NF-kappa B / Q000378:metabolism; D010455:Peptides / Q000235:genetics / Q000378:metabolism; D011401:Promoter Regions, Genetic; D014176:Protein Biosynthesis; D011487:Protein Conformation; D012333:RNA, Messenger / Q000378:metabolism; D012367:RNA, Viral / Q000378:metabolism; D013539:Simian virus 40 / Q000235:genetics; D015534:Trans-Activators / Q000235:genetics* / Q000378:metabolism; D015533:Transcriptional Activation; D054334:Viral Regulatory and Accessory Proteins
D003062:Codon; D016328:NF-kappa B; D010455:Peptides; D012333:RNA, Messenger; D012367:RNA, Viral; D015534:Trans-Activators; D054334:Viral Regulatory and Accessory Proteins; C057628:hepatitis B virus X protein