(1)H, (13)C and (15)N assignments of CdnL, an essential protein in Myxococcus xanthus

Mycobacterium tuberculosis CarD, an essential global transcriptional regulator forms amyloid-like fibrils

CarD is an essential global transcription regulator from Mycobacterium tuberculosis (Mtb) that binds RNA polymerase and activates transcription by stabilizing the transcription initiation complex. Available crystal structures have captured two distinct, monomeric and domain-swapped homodimeric, oligomeric states of CarD. However, the actual oligomeric state of CarD in solution and its biological relevance has remained unclear. Here, we confirm the presence of the homodimeric state of CarD in solution by using synchrotron-based small-angle X-ray scattering. Furthermore, by using biochemical and biophysical experiments, in addition to mass-spectrometry, transmission electron microscopy, and confocal imaging, we show that CarD is the first soluble cytosolic protein in Mtb which displays the tendency to form amyloid-like fibrils both in vitro as well as in vivo. We demonstrate that the deletion of the fourteen N-terminal residues involved in domain-swapping hampers amyloid formation, thus, suggesting that domain-swapping is crucial in amyloidogenesis. The discovery of the amyloidogenic property of an essential cytosolic global transcription regulator, CarD, in a pathogenic bacteria will further open up new frontiers in research.

Structure-function dissection of Myxococcus xanthus CarD N-terminal domain, a defining member of the CarD_CdnL_TRCF family of RNA polymerase interacting proteins

Two prototypes of the large CarD_CdnL_TRCF family of bacterial RNA polymerase (RNAP)-binding proteins, Myxococcus xanthus CarD and CdnL, have distinct functions whose molecular basis remain elusive. CarD, a global regulator linked to the action of several extracytoplasmic function (ECF) σ-factors, binds to the RNAP β subunit (RNAP-β) and to protein CarG via an N-terminal domain, CarDNt, and to DNA via an intrinsically unfolded C-terminal domain resembling eukaryotic high-mobility-group A (HMGA) proteins. CdnL, a CarDNt-like protein that is essential for cell viability, is implicated in σA-dependent rRNA promoter activation and interacts with RNAP-β but not with CarG. While the HMGA-like domain of CarD by itself is inactive, we find that CarDNt has low but observable ability to activate ECF σ-dependent promoters in vivo, indicating that the C-terminal DNA-binding domain is required to maximize activity. Our structure-function dissection of CarDNt reveals an N-terminal, five-stranded β -sheet Tudor-like domain, CarD1-72, whose structure and contacts with RNAP-β mimic those of CdnL. Intriguingly, and in marked contrast to CdnL, CarD mutations that disrupt its interaction with RNAP-β did not annul activity. Our data suggest that the CarDNt C-terminal segment, CarD61-179, may be structurally distinct from its CdnL counterpart, and that it houses at least two distinct and crucial function determinants: (a) CarG-binding, which is specific to CarD; and (b) a basic residue stretch, which is also conserved and functionally required in CdnL. This study highlights the evolution of shared and divergent interactions in similar protein modules that enable the distinct activities of two related members of a functionally important and widespread bacterial protein family.

Structural insights into RNA polymerase recognition and essential function of Myxococcus xanthus CdnL

CdnL and CarD are two functionally distinct members of the CarD_CdnL_TRCF family of bacterial RNA polymerase (RNAP)-interacting proteins, which co-exist in Myxococcus xanthus. While CarD, found exclusively in myxobacteria, has been implicated in the activity of various extracytoplasmic function (ECF) σ-factors, the function and mode of action of the essential CdnL, whose homologs are widespread among bacteria, remain to be elucidated in M. xanthus. Here, we report the NMR solution structure of CdnL and present a structure-based mutational analysis of its function. An N-terminal five-stranded β-sheet Tudor-like module in the two-domain CdnL mediates binding to RNAP-β, and mutations that disrupt this interaction impair cell growth. The compact CdnL C-terminal domain consists of five α-helices folded as in some tetratricopeptide repeat-like protein-protein interaction domains, and contains a patch of solvent-exposed nonpolar and basic residues, among which a set of basic residues is shown to be crucial for CdnL function. We show that CdnL, but not its loss-of-function mutants, stabilizes formation of transcriptionally competent, open complexes by the primary σA-RNAP holoenzyme at an rRNA promoter in vitro. Consistent with this, CdnL is present at rRNA promoters in vivo. Implication of CdnL in RNAP-σA activity and of CarD in ECF-σ function in M. xanthus exemplifies how two related members within a widespread bacterial protein family have evolved to enable distinct σ-dependent promoter activity.

Crystal structure of Mycobacterium tuberculosis CarD, an essential RNA polymerase binding protein, reveals a quasidomain-swapped dimeric structural architecture

Mycobacterium tuberculosis (Mtb) CarD is an essential transcriptional regulator that binds RNA polymerase and plays an important role in reprogramming transcription machinery under diverse stress conditions. Here, we report the crystal structure of CarD at 2.3 Å resolution, that represents the first structural description of CarD/CdnL-Like family of proteins. CarD adopts an overall bi-lobed structural architecture where N-terminal domain resembles 'tudor-like' domain and C-terminal domain adopts a novel five helical fold that lacks the predicted leucine zipper structural motif. The structure reveals dimeric state of CarD resulting from β-strand swapping between the N-terminal domains of each individual subunits. The structure provides crucial insights into the possible mode(s) of CarD/RNAP interactions.