Analysis of Viral Promoters for Transgene Expression and of the Effect of 5'-UTRs on Alternative Translational Start Sites in Chlamydomonas .

Microalgae biotechnology has the potential to produce high quality bioproducts in a sustainable manner. Here, has shown great potential as a host for biotechnological exploitation. However, low expression of nuclear transgenes is still a problem and needs to be optimized. In many model organisms, viral promoters are used to drive transgene expression at high levels. However, no viruses are known to infect , and known viral promoters are not functional. Recently, two different lineages of giant viruses were identified in the genomes of field isolates. In this work, we tested six potentially strong promoters from these viral genomes for their ability to drive transgene expression in . We used , , and as reporter genes, and three native benchmark promoters as controls. None of the viral promoters drove expression of any reporter gene beyond background. During our study, we found that mCherry variants are produced by alternative in-frame translational start sites in . We show that this problem can be overcome by mutating the responsible methionine codons to codons for leucine and by using the 5'-UTR of instead of the 5'-UTRs of or . Apparently, the 5'-UTR promotes the use of the first start codon. This could be mediated by the formation of a stem-loop between sequences of the 5'-UTR and sequences downstream of the first AUG in the reporter, potentially increasing the dwell time of the scanning 40S subunit on the first AUG and thus decreasing the probability of leaky scanning.