The physiology of L-methionine catabolism to the secondary metabolite ethylene by Escherichia coli

Abstract

Catabolism of L-methionine by Escherichia coli strain B SPAO led to the formation of ethylene as a secondary metabolite (ethylenogenesis). Methionine was initially deaminated by a transamination reaction to the 2-oxo acid 2-oxo-4-methylthiobutyric acid (KMBA) which was then converted to ethylene. The utilization of L-methionine as an additional nitrogen source was investigated by examining ethylene synthesis under different nitrogen supply conditions. Ethylene formation in batch culture was unaffected by the concentration of the precursor L-methionine in the medium although increasing concentrations of NH4Cl resulted in progressively less ethylene formation. Cultures grown without L-methionine did not produce ethylene but were able to synthesize ethylene when L-methionine or KMBA was provided. Addition of L-tyrosine to batch cultures reduced the yield of ethylene after 42 h by 54%. Under these conditions the maximum transient level of KMBA was reduced by 32% and occurred later compared to when L-methionine was the only amino acid supplement. Continuous cultures grown under ammonia limitation produced both ethylene and KMBA. In contrast, when glucose was limiting, neither of these metabolites were produced. Cells harvested from continuous cultures grown under glucose or ammonia limitation were able to synthesize ethylene from either L-methionine or KMBA although their capacity for ethylene synthesis (ethylenogenic capacity) was optimal under ammonia limitation (C:N ratio = 20).