Molecular Biology: Genome Organization and Expression


Papillomavirus encodes a double-stranded DNA genome. It is approximately 8,000 bp long and is monopartite and circular in nature. Replication and virion assembly occur in the host cell nucleus— notably, Papillomavirus does not encode its own viral DNA dependent DNA polymerase, so when the virus replicates it utilizes the host DNA polymerase.

Interestingly enough, in Papillomavirus all of the open reading frames are encoded on one strand of DNA, which means all of the viral genes are located on a single strand! This major is a crucial difference between Papillomaviridae and Polyomaviridae, and was a key reason why the Papovaviridae family was split into the two new taxa (see History and Taxonomy link for more info on the split).

Depending on the type of Papillomavirus, there are nine or ten open reading frames. Papillomavirus produces eight early proteins (named E1-E8) in addition to two capsid proteins, L1 and L2. The origin of replication is contained in a region called the LCR (stands for long control region) but is confusingly sometimes also reffered to as the upstream regulatory region (URR). This region is approximately 1 kbp long and in addition to containing the origin of DNA replication also encodes constitutive transcriptional enhancers.


Image Above: A Diagram of HPV16 Genome-- notice the early genes and late capsid gene products!

Transcription of Papillomavirus is complicated and tightly regulated in a temporal manner. The ten open reading frames are specified as either early or late: the early regions of the genome express viral regulatory proteins E1-E8, some of which are necessary for host cell transformation. The late region of the genome expresses the capsid proteins L1 and L2. In addition to the temporal regulation of transcription, Papillomaviruses have multiple promoters, exhibit alternative splicing, and the control of transcription is coordinated with the squamous host cell stage of differentiation.

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