Promoter activity of sequences located upstream of the human papillomavirus types of 16 and 18 late regions.

The regulation of human papillomavirus (HPV) late gene expression is difficult to analyse because the late proteins L1 and L2 are only produced in the upper layers of terminally differentiated keratinocytes. However, for the minor capsid protein L2 of HPV types 1, 6, 11 and 16, rare mRNAs or cDNAs starting 3' of the E5 open reading frame (ORF) were previously described. In order to analyse whether the DNA region preceding the late ORFs (late upstream region, LUR) of HPV-16 and HPV-18 has promoter activity, transient transfection assays employing luciferase reporter constructs were performed. The results show that the LUR of HPV-16 and HPV-18 exhibits an orientation-dependent promoter activity in different cells. By analysing 3'-deletion mutants of the HPV-16 LUR, we identified 78 bp within the sequence between the E5 and L2 ORFs to be critical for the promoter activity. Furthermore, the analysis of a 5'-deletion mutant revealed a negative cis-regulatory element located within the E2 ORF. The HPV-16 early poly(A) signal is located downstream of the critical promoter region. Inactivation of this element by site-directed mutagenesis strongly enhanced luciferase activity. However, mutation of two potential TATA-binding protein (TBP) sites located within the critical promoter region did not abolish the activity. Altogether, these data indicate the possibility of a TATA-less promoter in the HPV-16 and HPV-18 LURs. Together with the early poly(A) signal, this potential promoter might be involved in the differentiation-dependent regulation of late gene expression.