Human papillomavirus type 16 expresses a variety of alternatively spliced mRNAs putatively encoding the E2 protein

The full-length E2 protein of human papillomavirus type 16 is believed to act as a trans-repressor of the viral p97 promoter. Previous reports have provided evidence that transcripts with the potential to encode the E2 protein contain the 880/2708 splice junction. We have further analyzed the structure of the E2-encoding transcripts. Employing the RNA polymerase chain reaction (PCR) technique and analyses of the RNA PCR products by Southern blot hybridization and DNA sequencing, we revealed the existence of a variety of alternatively spliced mRNAs, with the capacity to encode the full-length E2 protein. Two novel splice junctions were identified at nucleotides 880/2581 and 226/2708. E2 mRNAs characterized by the 880/2581 splice junction contain sequences from the E1 orf predicted to encode a truncated E1 polypeptide consisting mainly of the C terminal amino acids. Transcripts with the 226/2708 splice junction could encode a novel E6 protein, designated E6IV, containing C terminal amino acids derived from an out-of-frame region of the E1 ORF. Three different E6-E7 exons were identified in mRNAs containing the 880/2708 and the 880/2581 splice junctions, namely, E6-E7, E6I-E7, E6II-E7. The E6I-E7 mRNAs are the most abundant. Expression of the various E2 mRNAs was detected in human keratinocytes immortalized by HPV16, in cervical tumors, and in carcinoma cell lines.