N-acetylmuramoyl-L-alanine amidase of Escherichia coli K12. Possible physiological functions

Abstract

Various experiments were carried out in an attempt to determine the possible physiological function of the N-acetylmuramoyl-L-alanine amidase purified from Escherichia coli K12 on the basis of its activity on N-acetylmuramoyl-L-alanyl-D-gamma-glutamyl-meso-diaminopimelic acid [MurNAc-LAla-DGlu(msA2pm)]. A Km value of 0.04 mM was determined with this substrate. Specificity studies revealed that compounds with a MurNAc-LAla linkage are the most probable substrates of this enzyme in vivo. Purified amidase had no effect on purified peptidoglycan and only low levels (1-2.5%) of cleaved MurNAc-LAla linkages were detected in peptidoglycan isolated from normally growing cells. However, the action of the amidase in vivo on peptidoglycan was clearly detectable during autolysis. The amidase activity of cells treated by osmotic shock, ether or toluene, as well as that of mutants with altered outer membrane composition was investigated. Attempts to reveal a transfer reaction catalysed by amidase were unsuccessful. Furthermore, by its location and specificity, amidase was clearly not involved in the formation of UDP-MurNAc. The possibility that it might be functioning in vivo as a hydrolase degrading exogeneous peptidoglycan fragments in the periplasma was substantiated by the fact that MurNAc itself and MurNAc-peptides could sustain growth of E. coli as sole carbon and nitrogen sources. Finally, out of 200 thermosensitive mutants examined for altered amidase activity, only two strains had less than 50% of the normal level of activity, whereas ten strains were found to possess more than 50%. In fact, two of the overproducers encountered presented a 4-5-fold increase in activity.