Cryo-EM Structures Reveal Transcription Initiation Steps by Yeast Mitochondrial RNA Polymerase.

Mitochondrial RNA polymerase (mtRNAP) is crucial in cellular energy production, yet understanding of mitochondrial DNA transcription initiation lags that of bacterial and nuclear DNA transcription. We report structures of two transcription initiation intermediate states of yeast mtRNAP that explain promoter melting, template alignment, DNA scrunching, abortive synthesis, and transition into elongation. In the partially melted initiation complex (PmIC), transcription factor MTF1 makes base-specific interactions with flipped non-template (NT) nucleotides "AAGT" at -4 to -1 positions of the DNA promoter. In the initiation complex (IC), the template in the expanded 7-mer bubble positions the RNA and NTP analog UTPαS, while NT scrunches into an NT loop. The scrunched NT loop is stabilized by the centrally positioned MTF1 C-tail. The IC and PmIC states coexist in solution, revealing a dynamic equilibrium between two functional states. Frequent scrunching/unscruching transitions and the imminent steric clashes of the inflating NT loop and growing RNA:DNA with the C-tail explain abortive synthesis and transition into elongation.

Human mitochondrial transcription factors TFAM and TFB2M work synergistically in promoter melting during transcription initiation.

Human mitochondrial DNA is transcribed by POLRMT with the help of two initiation factors, TFAM and TFB2M. The current model postulates that the role of TFAM is to recruit POLRMT and TFB2M to melt the promoter. However, we show that TFAM has 'post-recruitment' roles in promoter melting and RNA synthesis, which were revealed by studying the pre-initiation steps of promoter binding, bending and melting, and abortive RNA synthesis. Our 2-aminopurine mapping studies show that the LSP (Light Strand Promoter) is melted from -4 to +1 in the open complex with all three proteins and from -4 to +3 with addition of ATP. Our equilibrium binding studies show that POLRMT forms stable complexes with TFB2M or TFAM on LSP with low-nanomolar Kd values, but these two-component complexes lack the mechanism to efficiently melt the promoter. This indicates that POLRMT needs both TFB2M and TFAM to melt the promoter. Additionally, POLRMT+TFB2M makes 2-mer abortives on LSP, but longer RNAs are observed only with TFAM. These results are explained by TFAM playing a role in promoter melting and/or stabilization of the open complex on LSP. Based on our results, we propose a refined model of transcription initiation by the human mitochondrial transcription machinery.

Transcriptional fidelities of human mitochondrial POLRMT, yeast mitochondrial Rpo41, and phage T7 single-subunit RNA polymerases.

Single-subunit RNA polymerases (RNAPs) are present in phage T7 and in mitochondria of all eukaryotes. This RNAP class plays important roles in biotechnology and cellular energy production, but we know little about its fidelity and error rates. Herein, we report the error rates of three single-subunit RNAPs measured from the catalytic efficiencies of correct and all possible incorrect nucleotides. The average error rates of T7 RNAP (2 × 10-6), yeast mitochondrial Rpo41 (6 × 10-6), and human mitochondrial POLRMT (RNA polymerase mitochondrial) (2 × 10-5) indicate high accuracy/fidelity of RNA synthesis resembling those of replicative DNA polymerases. All three RNAPs exhibit a distinctly high propensity for GTP misincorporation opposite dT, predicting frequent A→G errors in RNA with rates of ∼10-4 The A→C, G→A, A→U, C→U, G→U, U→C, and U→G errors mostly due to pyrimidine-purine mismatches were relatively frequent (10-5-10-6), whereas C→G, U→A, G→C, and C→A errors from purine-purine and pyrimidine-pyrimidine mismatches were rare (10-7-10-10). POLRMT also shows a high C→A error rate on 8-oxo-dG templates (∼10-4). Strikingly, POLRMT shows a high mutagenic bypass rate, which is exacerbated by TEFM (transcription elongation factor mitochondrial). The lifetime of POLRMT on terminally mismatched elongation substrate is increased in the presence of TEFM, which allows POLRMT to efficiently bypass the error and continue with transcription. This investigation of nucleotide selectivity on normal and oxidatively damaged DNA by three single-subunit RNAPs provides the basic information to understand the error rates in mitochondria and, in the case of T7 RNAP, to assess the quality of in vitro transcribed RNAs.

Assembly and Cryo-EM structure determination of yeast mitochondrial RNA polymerase initiation complex intermediates.

In yeast mitochondria, transcription initiation requires assembly of mitochondrial RNA polymerase and transcription initiation factor MTF1 at the DNA promoter initiation site. This protocol describes the purification of the component proteins and assembly of partially melted and fully melted initiation complex states. Both states co-exist in equilibrium in the same sample as seen by cryoelectron microscopy (cryo-EM) and allow elucidation of MTF1's structural roles in controlling the transition into elongation. We further outline how analysis of the complex by light scattering, thermal shift assay, and ultrafiltration assay exhibits reproducible results. For complete details on the use and execution of this protocol, please refer to De Wijngaert et al. (2021).

Helicase promotes replication re-initiation from an RNA transcript.

To ensure accurate DNA replication, a replisome must effectively overcome numerous obstacles on its DNA substrate. After encountering an obstacle, a progressing replisome often aborts DNA synthesis but continues to unwind. However, little is known about how DNA synthesis is resumed downstream of an obstacle. Here, we examine the consequences of a non-replicating replisome collision with a co-directional RNA polymerase (RNAP). Using single-molecule and ensemble methods, we find that T7 helicase interacts strongly with a non-replicating T7 DNA polymerase (DNAP) at a replication fork. As the helicase advances, the associated DNAP also moves forward. The presence of the DNAP increases both helicase's processivity and unwinding rate. We show that such a DNAP, together with its helicase, is indeed able to actively disrupt a stalled transcription elongation complex, and then initiates replication using the RNA transcript as a primer. These observations exhibit T7 helicase's novel role in replication re-initiation.

Fluorescent methods to study transcription initiation and transition into elongation.

The DNA-dependent RNA polymerases induce specific conformational changes in the promoter DNA during transcription initiation. Fluorescence spectroscopy sensitively monitors these DNA conformational changes in real time and at equilibrium providing powerful ways to estimate interactions in transcriptional complexes and to assess how transcription is regulated by the promoter DNA sequence, transcription factors, and small ligands. Ensemble fluorescence methods described here probe the individual steps of promoter binding, bending, opening, and transition into the elongation using T7 phage and mitochondrial transcriptional systems as examples.

Translation initiation factor 4B homodimerization, RNA binding, and interaction with Poly(A)-binding protein are enhanced by zinc.

The eukaryotic translation initiation factor (eIF) 4B promotes the RNA-dependent ATP hydrolysis activity and ATP-dependent RNA helicase activity of eIF4A and eIF4F during translation initiation. eIF4B also helps to organize the assembly of the translational machinery through its interactions with eIF4A, eIF4G, eIF3, the poly(A)-binding protein (PABP), and RNA. Although the function of eIF4B is conserved among plants, animals, and yeast, eIF4B is one of the least conserved of initiation factors at the sequence level. Mammalian eIF4B is a constitutive dimer; however, conflicting reports have suggested that plant eIF4B may exist as a monomer or a dimer. In this study, we show that eIF4B from wheat can form a dimer and we identify the region responsible for its dimerization. Zinc stimulated homodimerization of eIF4B and bound eIF4B with a Kd of 19.7 nM. Zinc increased the activity of the eIF4B C-terminal RNA-binding domain specifically. Zinc promoted the interaction between eIF4B and PABP but not the interaction between eIF4B and eIF4A or eIFiso4G, demonstrating that the effect of zinc was highly specific. The interaction between PABP and eIFiso4G was also stimulated by zinc but required significantly higher levels of zinc. Interestingly zinc abolished the ability of eIFiso4G to compete with eIF4B in binding to their overlapping binding sites in PABP by preferentially promoting the interaction between eIF4B and PABP. Our observations suggest that wheat eIF4B can dimerize but requires zinc. Moreover zinc controls the partner protein selection of PABP such that the interaction with eIF4B is preferred over eIFiso4G.