Total synthesis of a gene for octopus rhodopsin and its preliminary expression.
To carry out systematic structure-function studies of octopus rhodopsin, photoreceptor protein of octopus visual cells, by means of specific amino-acid replacements, we have totally synthesized a DNA duplex of 1,365 base pairs that encodes the entire octopus rhodopsin of 455 amino acids [Ovchinnikov et al. (1988) FEBS Lett. 232, 69-72] by introducing codons preferred in Escherichia coli. Total synthesis simplifies site-specific mutagenesis in all parts of the gene by replacement of short restriction fragments by their newly synthesized counterparts containing the required nucleotide alterations. Thirty unique restriction sites were introduced in the octopus rhodopsin gene, which was assembled on a plasmid in two steps. Five cartridge genes of 344, 296, 320, 212, and 317 base pairs capable of being expressed independently were first constructed by using 48 synthetic oligonucleotides ranging in size from 54 to 73 nucleotides. The entire gene was constructed by consecutive linkage of cartridge genes. These cartridge genes were designed to correspond to the transmembrane helical unit of octopus rhodopsin, resulting in easy construction of various chimeric rhodopsins. The nucleotide sequences were confirmed by sequencing the cartridges as well as the entire gene. These synthetic genes were cloned into an expression vector carrying the trp promoter of E. coli, and were preliminarily expressed in vitro and in vivo.