Enhanced expression in tobacco of the gene encoding green fluorescent protein by modification of its codon usage.

The gene encoding green fluorescent protein (GFP) from Aequorea victoria was resynthesized to adapt its codon usage for expression in plants by increasing the frequency of codons with a C or a G in the third position from 32 to 60%. The strategy for constructing the synthetic gfp gene was based on the overlap extension PCR method using 12 long oligonucleotides as the starting material and as primers. The new gene contains 101 silent nucleotide changes compared to its wild-type counterpart used in this study. Several transgenic tobacco lines containing the wild-type gfp gene contained minute amounts of a smaller protein cross-reacting with GFP antiserum, whereas only one protein of the expected size was found in transgenics with the synthetic gfp gene. The smaller protein was probably encoded by a truncated gfp mRNA created by splicing of a 84 bp cryptic intron as detected by a reverse transcription-PCR technique. A comparison of GFP production in transgenics with the wild-type and the synthetic gfp gene under the control of the enhanced CaMV 35S promoter showed that the large-scale alterations in the gfp gene increased the frequency of high expressors in the transgenic population but hardly changed the maximum GFP concentrations. The latter phenomenon may be attributed to a reduced regeneration capacity of transformed cells with higher GFP concentrations.