Alternative sulfur acquisition pathways in Neisseria gonorrhoeae

Abstract

Neisseria gonorrhoeae is an obligate human pathogen responsible for the sexually transmitted infection, gonorrhoea. Its success as a pathogen is partly due to robust defence mechanisms that provide protection against oxidative stress encountered during infection. Reduced sulfur compounds such as glutathione, cysteine and methionine are integral to this response and pathogenic growth.

Due to a large genomic deletion and pseudogenes, N. gonorrhoeae is incapable of sulfur acquisition via traditional routes and therefore cannot grow when sulfate is the sole sulfur source. However, N. gonorrhoeae can grow in the presence of thiosulfate but lacks the ability to reduce thiosulfate via the conventional thiosulfate reduction pathway. This raises questions of how N. gonorrhoeae acquires sulfur for cysteine biosynthesis?

We have identified two sulfurtransferase enzymes (Str and PspE) in N. gonorrhoeae that we hypothesise provide sulfur in the form needed for cysteine synthesis. We show these enzymes have thiosulfate-thiol sulfurtransferase activity and, importantly, produce sulfide that could be utilised for cysteine biosynthesis. Furthermore, we demonstrate that Str is a promiscuous enzyme with respect to thiol acceptor substrates and, intriguingly, is capable of cyanide detoxification. Our N. gonorrhoeae sulfurtransferase deletion strain has a reduced ability to grow when thiosulfate is the only available sulfur source, supporting our hypothesis that Str utilises exogenous inorganic thiosulfate. However, due to functional redundancy provided by the presence of the second sulfurtransferase, PspE, construction of a double knockout strain is essential in understanding the full effect of these enzymes in relation to pathogenicity.

Our proposed energetically favourable pathway of thiosulfate reduction via sulfurtransferase enzymes could be pivotal in advancing our understanding of how pathogens fulfil their sulfur requirements. However, much is to be elucidated regarding the role of these ubiquitous enzymes in bacterial pathogens. Herein, this thesis offers insight into the versatility, function, and formal mechanisms of sulfurtransferases within N. gonorrhoeae.