The Diverse Functions Of Nitric Oxide Synthases In Prokaryotes

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Nitric Oxide Synthases (NOSs) are highly regulated enzymes that oxidize Larginine to synthesize the cytotoxin and signaling molecule nitric oxide (NO). NO in mammals is involved in many cellular process ranging from regulation of blood pressure to protection against pathogens; however, the roles of homologous prokaryotic NOSs are just beginning to be revealed. In Streptomyces turgidiscabies NOS supplies NO to produce 4-nitro-tryptophan which is a component of thaxtomin, a plant toxin responsible for the potato scab disease. In Bacillus subtilis, NO appears to protect against oxidative stress. In this work I investigated the function of NOS in Deinococcus radiodurans (D. radiodurans), a bacterium that withstands desiccation, reactive oxygen species, and doses of radiation that would be lethal to most organisms. Deletion of the nos gene severely compromises the recovery of D. radiodurans from ultraviolet (UV) radiation damage. The delta-nos defect can be complemented with recombinant NOS, rescued by exogenous nitric oxide (NO) and mimicked in the wildtype strain with an NO scavenging compound. UV radiation induces the upregulation of the nos gene and cellular NO production on similar time scales. NOS-derived NO upregulates transcription of obgE, a gene involved in bacterial growth proliferation and stress response. Overexpression of the ObgE GTPase in the delta-nos background substantially alleviates the growth defect after radiation damage. Thus, NO acts as a signal for transcriptional regulation for growth proliferation in D. radiodurans. However, there is evidence of additional roles of NOS in D. radiodurans.. It has been shown previously that DrNOS interacts with tryptophanyl-tRNA synthetase II (TrpRS II), and DrNOS selectively nitrates tryptophan at the 4-position and TrpRS II couples this species to tRNATrp. I have studied the effect of ∆nos on tRNATrp pools in D. radiodurans. These studies have found that D. radiodurans contains at least two forms of tRNATrp, whose presence depend upon the nos gene and the growth phase of the bacteria. One of the forms shows acid sensitivity, where as the other does not. Gene knockouts of both trpRS I and trpRS II in D. radiodurans are viable although growth of ∆trpRS II is compromised under stress. No link was found between NOSassociated tRNA modification and the ability to withstand radiation damage. Thus, in bacteria as in mammals, NOS appears to have multiple functions in D. radiodurans.

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