Optimizing the secondary structure of human papillomavirus type 16 L1 mRNA enhances L1 protein expression in Saccharomyces cerevisiae.

Human papillomavirus (HPV) type 16 is the most significant type in the prophylactic vaccine field because 50-60% of cervical cancer cases are caused by infection with HPV type 16. The L1 protein, which is the highly immunogenic major capsid protein of HPV, is the major component of the prophylactic vaccine. To enhance L1 protein expression in Saccharomyces cerevisiae we used a strategy involving the modification of the HPV16 L1 gene (MO) to reduce the amount of predicted mRNA secondary structure, rather than a strategy aiming at optimizing codon bias as has often been used to enhance the expression of foreign genes. The mean expression level of L1 protein in MO-transformed candidates was four-fold higher than in cells transformed with the wild type HPV16 L1 gene (WT), while the fraction of L1 mRNA transcribed from MO in total RNA was four-fold lower than with L1 mRNA transcribed from WT. This demonstrates that L1 protein expression in MO-transformed cells was increased by a post-transcriptional mechanism. The L1 protein produced in MO-transformed cells self-assembled into virus-like particles (VLP) with features similar to those of the native virion. These results indicate that the strategy of optimizing the secondary structure of HPV16 L1 mRNA was successful in increasing L1 protein expression in Saccharomyces cerevisiae.