Abstract
BackgroundDown-regulation or silencing of transgene expression can be a major hurdle to both molecular studies and biotechnology applications in many plant species. Sugarcane is particularly effective at silencing introduced transgenes, including reporter genes such as the firefly luciferase gene.Synthesizing transgene coding sequences optimized for usage in the host plant is one method of enhancing transgene expression and stability. Using specified design rules we have synthesised new coding sequences for both the firefly luciferase and Renilla luciferase reporter genes. We have tested these optimized versions for enhanced levels of luciferase activity and for increased steady state luciferase mRNA levels in sugarcane.ResultsThe synthetic firefly luciferase (luc*) and Renilla luciferase (Renluc*) coding sequences have elevated G + C contents in line with sugarcane codon usage, but maintain 75% identity to the native firefly or Renilla luciferase nucleotide sequences and 100% identity to the protein coding sequences.Under the control of the maize pUbi promoter, the synthetic luc* and Renluc* genes yielded 60x and 15x higher luciferase activity respectively, over the native firefly and Renilla luciferase genes in transient assays on sugarcane suspension cell cultures.Using a novel transient assay in sugarcane suspension cells combining co-bombardment and qRT-PCR, we showed that synthetic luc* and Renluc* genes generate increased transcript levels compared to the native firefly and Renilla luciferase genes.In stable transgenic lines, the luc* transgene generated significantly higher levels of expression than the native firefly luciferase transgene. The fold difference in expression was highest in the youngest tissues.ConclusionsWe developed synthetic versions of both the firefly and Renilla luciferase reporter genes that resist transgene silencing in sugarcane. These transgenes will be particularly useful for evaluating the expression patterns conferred by existing and newly isolated promoters in sugarcane tissues. The strategies used to design the synthetic luciferase transgenes could be applied to other transgenes that are aggressively silenced in sugarcane.
Highlights
Down-regulation or silencing of transgene expression can be a major hurdle to both molecular studies and biotechnology applications in many plant species
We took advantage of the redundancy of the genetic code to design new sequences encoding for synthetic versions of luc and Renilla luciferase (Renluc), and to eliminate rare codons, strings of A or T bases, palindromes, polyadenylation signals, and to avoid the potential addition of an intron
In conclusion, we demonstrate that designing transgenes to mimic codon usage in sugarcane and avoid known destabilizing motifs is an effective strategy to optimise transgene expression and prevent transgene silencing in sugarcane
Summary
Down-regulation or silencing of transgene expression can be a major hurdle to both molecular studies and biotechnology applications in many plant species. Sugarcane is effective at silencing introduced transgenes, including reporter genes such as the firefly luciferase gene. Synthesizing transgene coding sequences optimized for usage in the host plant is one method of enhancing transgene expression and stability. Using specified design rules we have synthesised new coding sequences for both the firefly luciferase and Renilla luciferase reporter genes. We have tested these optimized versions for enhanced levels of luciferase activity and for increased steady state luciferase mRNA levels in sugarcane. The GFP reporter gene from Aequorea victoria is not expressed well in certain plant species. Little or no GFP fluorescence could be detected in Arabidopsis or tobacco transgenic plants despite the use of strong promoters such as CaMV 35S [6,7]
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