SUPPRESSION OF ESCHERICHIA-COLI FORMATE HYDROGENLYASE ACTIVITY BY TRIMETHYLAMINE N-OXIDE IS DUE TO DRAINAGE OF THE INDUCER FORMATE

The effect of the addition of trimethylamine N-oxide (TMAO) in the gro wth medium on Escherichia coli anaerobic fermentative and respiratory pathways was examined. Formate dehydrogenase H (FDH-H) activity was to tally repressed by the addition of 40 mM TMAO, whereas the overall hyd rogenase (HYD) activity was reduced by 25%. Accordingly, expression of lacZ operon fusions with the fdhF and hycB structural genes specifyin g FDH-H and HYD3 was reduced sevenfold and eightfold, respectively, le ading to suppression of an active formate hydrogenlyase system, In con trast, global respiratory formate-dependent phenazine methosulphate re ductase (FDH-PMS) activity, which consists of both the major anaerobic FDH-N enzyme and the aerobic FDH-Z isoenzyme, was increased approxima tely twofold. This was corroborated by a 2.5-fold stimulation of the s ole fdoG-uidA transcriptional fusion which reflects the synthesis of t he respiratory aerobic FDH-Z enzyme. In fdhD, fdhE or torA mutants lac king either FDH-PMS activity or TMAO reductase (TOR) activity, the for mate hydrogenlyase pathway was no longer inhibited by TMAO. In additio n, introduction of 30 mM formate in the growth medium was found to rel ieve the repressive effect of TMAO in the wild-type strain. When TMAO was added as terminal electron acceptor a significant enhancement of a naerobic growth was observed with the wild-type strain and the fdoG mu tant. It was associated with the concomitant suppression of the format e hydrogenlyase enzymes. This was in contrast to the fdnG and torA mut ants whose growth pattern and fermentative enzymes remained unaffected . Taken together, these results strongly suggest that formate-dependen t reduction of TMAO via FDH-N and TOR reduces the amount of formate av ailable for induction of the formate hydrogenlyase pathway.