Abstract
Human papillomavirus 18 (HPV18) E6 and E7 oncogenes are transcribed as a single bicistronic E6E7 pre-mRNA. The E6 ORF region in the bicistronic E6E7 pre-mRNA contains an intron. Splicing of this intron disrupts the E6 ORF integrity and produces a spliced E6*I RNA for efficient E7 translation. Here we report that the E6 intron has two overlapped branch point sequences (BPS) upstream of its 3′ splice site, with an identical heptamer AACUAAC, for E6*I splicing. One heptamer has a branch site adenosine (underlined) at nt 384 and the other at nt 388. E6*I splicing efficiency correlates to the expression level of E6 and E7 proteins and depends on the selection of which branch site. In general, E6*I splicing prefers the 3′ss-proximal branch site at nt 388 over the distal branch site at nt 384. Inactivation of the nt 388 branch site was found to activate a cryptic acceptor site at nt 636 for aberrant RNA splicing. Together, these data suggest that HPV18 modulates its production ratio of E6 and E7 proteins by alternative selection of the two mapped branch sites for the E6*I splicing, which could be beneficial in its productive or oncogenic infection according to the host cell environment.
Highlights
Human papillomaviruses (HPV) are small, non-enveloped DNA viruses and contain a double-stranded DNA genome *8 kb in size
We discovered that the Human papillomavirus 18 (HPV18) E6 intron contains two alternative branch sites at nt 384 and 388, but preferentially uses a 30ss-proximal nt 388 adenosine as a branch site for E6E7 pre-mRNA splicing
We found that five heptamer sequences with an A at the 6th position exhibited a consensus value (CV) above 65 that resemble to a branch point sequences (BPS) motif (Fig. 1C and 1D)
Summary
Human papillomaviruses (HPV) are small, non-enveloped DNA viruses and contain a double-stranded DNA genome *8 kb in size. Escaping of the E6 intron from RNA splicing leads to remain the E6 ORF integrity and is necessary for E6 protein translation. Splicing of the E6 intron during HR-HPV infection is highly efficient and is required for E7 protein translation (Zheng et al 2004; Zheng and Baker 2006; Tang et al 2006). Majority of the spliced product with a disrupted E6 ORF is the E6*I which serves as an E7 mRNA for E7 protein translation (Tang et al 2006). It remains unclear how the E6 intron splicing is Virologica Sinica regulated for E7 expression, and how the E6 intron could escape from RNA splicing to express E6 protein during
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