The NADH oxidase of Streptococcus pneumoniae: its involvement in competence and virulence

A soluble flavoprotein that reoxidizes NADH and reduces molecular oxygen to water was purified from the facultative anaerobic human pathogen Streptoco ccus pneumoniae, The nucleotide sequence of nox, the gene which encodes it, has been determined and was characterized at the functional and physiologi cal level, Several nox mutants were obtained by insertion, nonsense or miss ense mutation. In extracts from these strains, no NADH oxidase activity cou ld be measured, suggesting that a single enzyme encoded by nox, having a C4 4 in its active site, was utilizing O-2 to oxidize NADH in S. pneumoniae. T he growth rate and yield of the NADH oxidase-deficient strains were not cha nged under aerobic or anaerobic conditions but the efficiency of developmen t of competence for genetic transformation during growth was markedly alter ed, Conditions that triggered competence induction did not affect the amoun t of Nox, as measured using Western blotting, indicating that nox does not belong to the competence-regulated genetic network. The decrease in compete nce efficiency due to the nox mutations was similar to that due to the abse nce of oxygen in the nox(+) strain, suggesting that input of oxygen into th e metabolism via NADH oxidase was important for controlling competence deve lopment throughout growth. This was not related to regulation of nox expres sion by O-2. Interestingly, the virulence and persistence In mice of a bloo d isolate was attenuated by a nox insertion mutation. Global cellular respo nses of S. pneumoniae, such as competence for genetic exchange or virulence in a mammalian host, could thus be modulated by oxygen via the NADH oxidas e activity of the bacteria, although the bacterial energetic metabolism is essentially anaerobic. The enzymatic activity of the NADH oxidase coded by nox was probably involved in transducing the external signal, corresponding to O-2 availability, to the cell metabolism and physiology; thus, this enz yme may function as an oxygen sensor. This work establishes, for the first time, the role of O-2 in the regulation of pneumococcal transformability an d virulence.