Codon optimization of papillomavirus genes

Early and late genes of human and animal papillomaviruses show a codon composition seemingly unfavorable for expression in mammalian cells. It remains unclear how the viruses manage to achieve high levels of late gene expression during the viral life cycle. One possible solution could be that the availability of certain t-RNAs changes with progressing stages of cellular differentiation. Previous studies have demonstrated that modification of codon usage of papillomavirus late (L1 and L2) and early genes (E7) can overcome poor expression of these proteins both in transient and in stable expression systems. This was shown not only for human but also for plant cells. Two strategies can be employed to alter codon usage: elimination of only those codons that are rarely used in a particular expression system, or exchange of all possible codons by the ones most frequently used. Currently, there are two protocols for codon modification--a template-less polymerase chain reaction (PCR)-based protocol, in which very long overlapping oligodeoxynucleotides are used in an overlap-extension reaction, or a ligase chain reaction, in which shorter oligodeoxynucleotides are fused together after an annealing procedure. Both methods are presented and discussed.