Proc. Natl. Acad. Sci. USA doi:10.1073/pnas.1422387112

Credit: PNAS

Mycobacterium tuberculosis, the causative agent of tuberculosis, can enter a dormant state within macrophages that is difficult to eliminate and can subsequently cause active infections. Environmental conditions such as hypoxia are thought to be key to initiating dormancy, which is characterized in part by greatly diminished protein synthesis. One of the genes most highly upregulated under these conditions is Rv1738, which encodes a 94-residue protein of unknown function. Bunker et al. envisioned that a crystal structure could be informative about its physiological role. However, crystals could not be obtained with the wild-type sequence or with sequences modified at the termini or using other common 'rescue' strategies. From prior experience, the authors suspected that crystallizing the protein as a racemate would be helpful because additional space groups would be accessible. The authors thus prepared the L- and D-protein mirror image forms of Rv1738 by total chemical synthesis. The racemic mixture rapidly formed crystals that diffracted to high resolution, and the structure was solved using computational methods that exploited the centrosymmetric crystal form. The structure shows that Rv1738 is a dimer with a six-stranded β-sheet wrapped around two α-helices. Within the crystal, hydrogen bonds between the β-sheet of the L- and D-protein forms create an extended ribbon, explaining why the racemic mixture crystallized but the native L-protein did not. With the structure in hand, the authors were able to identify homologs among the S30Ae ribosome-associated proteins, a family that includes cold-shock proteins from Escherichia coli and Vibrio cholerae that inhibit translation during environmental stresses. Modeling further suggested that structural variations between Rv1738 and S30Ae representatives could be accommodated within the ribosomal S30Ae binding site. These data suggest that Rv1738 contributes to the shutdown of ribosomal protein synthesis during the onset of dormancy, and set the stage for more directed study of this activity.