RESUMO
In human mitochondria, transcription termination events at a G-quadruplex region near the replication origin are thought to drive replication of mtDNA by generation of an RNA primer. This process is suppressed by a key regulator of mtDNA-the transcription factor TEFM. We determined the structure of an anti-termination complex in which TEFM is bound to transcribing mtRNAP. The structure reveals interactions of the dimeric pseudonuclease core of TEFM with mobile structural elements in mtRNAP and the nucleic acid components of the elongation complex (EC). Binding of TEFM to the DNA forms a downstream "sliding clamp," providing high processivity to the EC. TEFM also binds near the RNA exit channel to prevent formation of the RNA G-quadruplex structure required for termination and thus synthesis of the replication primer. Our data provide insights into target specificity of TEFM and mechanisms by which it regulates the switch between transcription and replication of mtDNA.
Assuntos
Replicação do DNA , DNA Mitocondrial/genética , Quadruplex G , Proteínas Mitocondriais/metabolismo , Fatores de Transcrição/metabolismo , Transcrição Gênica , Sequência de Aminoácidos , DNA Mitocondrial/química , Humanos , Mitocôndrias/metabolismo , Proteínas Mitocondriais/química , Modelos Moleculares , Elongação da Transcrição Genética , Fatores de Transcrição/química , Terminação da Transcrição GenéticaRESUMO
Here we report the crystal structure of the human mitochondrial RNA polymerase (mtRNAP) transcription elongation complex, determined at 2.65-Å resolution. The structure reveals a 9-bp hybrid formed between the DNA template and the RNA transcript and one turn of DNA both upstream and downstream of the hybrid. Comparisons with the distantly related RNA polymerase (RNAP) from bacteriophage T7 indicates conserved mechanisms for substrate binding and nucleotide incorporation but also strong mechanistic differences. Whereas T7 RNAP refolds during the transition from initiation to elongation, mtRNAP adopts an intermediary conformation that is capable of elongation without refolding. The intercalating hairpin that melts DNA during T7 RNAP initiation separates RNA from DNA during mtRNAP elongation. Newly synthesized RNA exits toward the pentatricopeptide repeat (PPR) domain, a unique feature of mtRNAP with conserved RNA-recognition motifs.
Assuntos
RNA Polimerases Dirigidas por DNA/química , Cristalografia por Raios X , DNA Mitocondrial/química , Humanos , Conformação Proteica , RNA/química , RNA MitocondrialRESUMO
Most thiopeptide antibiotics target the translational machinery: thiostrepton (ThS) and nosiheptide (NoS) target the ribosome and inhibit translation factor function, whereas GE2270A/T binds to the elongation factor EF-Tu and prevents ternary complex formation. We have used several in vitro translational machinery assays to screen a library of thiopeptide antibiotic precursor compounds and identified four families of precursor compounds that are either themselves inhibitory or are able to relieve the inhibitory effects of ThS, NoS, or GE2270T. Some of these precursors represent distinct compounds with respect to their ability to bind to ribosomes. The results not only provide insight into the mechanism of action of thiopeptide compounds but also demonstrate the potential of such assays for identifying lead compounds that might be missed using conventional inhibitory screening protocols.
