RESUMO
The stringent response, regulated by the bifunctional (p)ppGpp synthetase/hydrolase Rel in mycobacteria, is critical for long-term survival of the drug-tolerant dormant state of Mycobacterium tuberculosis. During amino acid starvation, MtRel senses a drop in amino acid concentration and synthesizes the messengers pppGpp and ppGpp, collectively called (p)ppGpp. Here, we investigate the role of the regulatory 'Aspartokinase, Chorismate mutase and TyrA' (ACT) domain in MtRel. Using NMR spectroscopy approaches, we report the high-resolution structure of dimeric MtRel ACT which selectively binds to valine out of all other branched-chain amino acids tested. A set of MtRel ACT mutants were generated to identify the residues required for maintaining the head-to-tail dimer. Through NMR titrations, we determined the crucial residues for binding of valine and show structural rearrangement of the MtRel ACT dimer in the presence of valine. This study suggests the direct involvement of amino acids in (p)ppGpp accumulation mediated by MtRel independent to interactions with stalled ribosomes. Database Structural data are available in the PDB database under the accession number 6LXG.
Assuntos
Aspartato Quinase/genética , Corismato Mutase/genética , Ligases/genética , Mycobacterium tuberculosis/genética , Aspartato Quinase/química , Aspartato Quinase/ultraestrutura , Corismato Mutase/química , Corismato Mutase/ultraestrutura , Guanosina Tetrafosfato/genética , Hidrolases/genética , Ligases/química , Ligases/ultraestrutura , Espectroscopia de Ressonância Magnética , Mycobacterium tuberculosis/patogenicidade , Domínios Proteicos/genética , Multimerização Proteica , Fatores de Transcrição/genéticaRESUMO
The cDNA and the chromosomal locus of the aroC gene of Aspergillus nidulans were cloned and is the first representative of a filamentous fungal gene encoding chorismate mutase (EC 5.4.99.5), the enzyme at the first branch point of aromatic amino acid biosynthesis. The aroC gene complements the Saccharomyces cerevisiae aro7Delta as well as the A. nidulans aroC mutation. The gene consists of three exons interrupted by two short intron sequences. The expressed mRNA is 0.96 kilobases in length and aroC expression is not regulated on the transcriptional level under amino acid starvation conditions. aroC encodes a monofunctional polypeptide of 268 amino acids. Purification of this 30-kDa enzyme allowed determination of its kinetic parameters (k(cat) = 82 s(-1), n(H) = 1. 56, [S](0.5) = 2.3 mM), varying pH dependence of catalytic activity in different regulatory states, and an acidic pI value of 4.7. Tryptophan acts as heterotropic activator and tyrosine as negative acting, heterotropic feedback-inhibitor with a K(i) of 2.8 microM. Immunological data, homology modeling, as well as electron microscopy studies, indicate that this chorismate mutase has a dimeric structure like the S. cerevisiae enzyme. Site-directed mutagenesis of a crucial residue in loop220s (Asp(233)) revealed differences concerning the intramolecular signal transduction for allosteric regulation of enzymatic activity.
