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Microbiology 154 (2008), 2796-2803; DOI  10.1099/mic.0.2008/020933-0
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Microbiology 154 (2008), 2796-2803; DOI  10.1099/mic.0.2008/020933-0
© 2008 Society for General Microbiology

Moonlighting function of glutamate racemase from Mycobacterium tuberculosis: racemization and DNA gyrase inhibition are two independent activities of the enzyme

Sugopa Sengupta1, Soumitra Ghosh1 and Valakunja Nagaraja1,2

1 Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore 560012, India
2 Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India

Correspondence
Valakunja Nagaraja
vraj{at}mcbl.iisc.ernet.in

Glutamate racemase (MurI) provides D-glutamate, a key building block in the peptidoglycan of the bacterial cell wall. Besides having a crucial role in cell wall biosynthesis, MurI proteins from some bacteria have been shown to act as an inhibitor of DNA gyrase. Mycobacterium tuberculosis and Mycobacterium smegmatis MurI exhibit these dual characteristics. Here, we show that the two activities of M. tuberculosis MurI are unlinked and independent of each other. The racemization function of MurI is not essential for its gyrase-inhibitory property. MurI–DNA gyrase interaction influences gyrase activity but has no effect on the racemization activity of MurI. Overexpression of MurI in vivo provides resistance to the action of ciprofloxacin, suggesting the importance of the interaction in gyrase modulation. We propose that the moonlighting activity of MurI has evolved more recently than its racemase function, to play a transient yet important role in gyrase modulation.

Abbreviations: AMF, autocrine motility factor; CD, circular dichroism; PGI, phosphoglucose isomerase

Three supplementary figures are available with the online version of this paper.






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