Studies on the biosynthesis of gramicidin S in a cell-free system from Bacillus brevis. Further attempts to elucidate its mechanism of synthesis

Abstract

The pH optima for the incorporation of 14c-labelled amino acids into gramicidin S by an 11000 g cell-free extract from B. brevis were determined. The pH optima for leucine, proline, phenylalanine, ornithine and valine were 7.5-7.7, 7.5-7.7, 7.7-7.9, 7.7-7.9 and 8.0-8.2 respectively. Hence the greatest difference in pH optima existed between leucine and valine, where it was 0.5 pH unit. The 11000 g cell-free extract incorporated into gramicidin S only the L-isomers of valine, proline and ornithine. However, both isomers of leucine are utilized and the experiments indicate that a leucine racemase exists in the 11000 g cell-free extract. With phenylalanine the L-isomer is utilized much more effectively than the D-isomer. This is noteworthy since it is the D-isomer that occurs in gramicidin S.. The experiments indicate thaF conversion of the L-isomer into the D-form takes place at a stage beyond that of the free amino acid.