First artificial gene synsized by Khorana was 

RNA synthetase is also known as RNA replicase which catalyzes RNA synthesis in virus-infected host cells. Restriction endonucleases recognize specific sequences in the DNA and cut the DNA into fragments. The nif genes encode enzymes involved in the fixation of atmospheric nitrogen into a form of nitrogen which is available to living organisms. H. G. Khorana and co-workers synthesized the gene for E. coli tyrosine tRNA precursor. The tyrosine tRNA precursor has 126 nucleotides. A complete sequence of dsDNA coding for tyrosine tRNA precursor of E. coli was synthesized. He then synthesized biologically functional gene coding for tyrosine t-RNA of E.coil. It was 207 base pairs long with 51 bp long promoter region, a 126 bp long precursor region of tRNA, a 25 bp long DNA in which 16 base pairs contained the restriction site for Eco RI. Option D is the correct answer.