gramicidin s biosynthesis operon containing the structural genes grsa and grsb has an open reading frame encoding a protein homologous to fatty acid thioesterases the dna sequence of about 59 kilobase pairs kbp of the gramicidin s biosynthesis operon grs was determined three open reading frames were identified the corresponding genes called grst grsa and grsb were found to be organized in one transcriptional unit not two as previously reported m krause and m a marahiel j bacteriol 17046694674 1988 the entire nucleotide sequence of grsa coding for the 126663kilodalton gramicidin s synthetase 1 grst encoding a 29191kilodalton protein of unknown function and 732 bp of the 5 end of grsb encoding the gramicidin s synthetase 2 were determined a single initiation site of transcription 81 bp upstream of the grst initiation condon gtg was identified by highresolution s1 mapping studies the sequence of the grsa gene product showed a high degree of homology to the tyrocidine synthetase 1 tyca protein and that of grst exhibited a significant degree of homology to vertebrate fatty acid thioesterases