Single Molecule and Synthetic Biology Studies of Transcription
The horizons of biology are ever expanding, from the discernment of the detailed mechanisms of enzyme function, to the manipulation of the physiological processes of whole organisms and ecosystems. Single molecule studies allow for the characterization of the individual processes that comprise an enzyme's mechanochemical cycle. Through standardization and generalization of biological techniques, components, and knowledge, synthetic biology seeks to expand the scale of biological experiments and to usher in an age of biology as a true engineering science, in which those studying different hierarchical levels of sophistication need not start from the fundamental biochemical principles underlying all biological experiments. Here we report our findings on the processes governing transcription and its role in gene expression through the use of both single molecule and synthetic biology methods.
We have established a promoter-free, factor-free method of initiation of transcription by the mitochondrial RNA polymerase in Saccharomyces cerevisiae, Rpo41 through the use of synthetic oligonucleotides to imitate the hybridization geometry of Rpo41 during active transcription. Using this system, we have established that a sub-micromolar NTP concentration is appropriate for non-saturating transcriptional runoff assays. We have optimized the transcription buffer and found that 10 mM MgCl2, 40 mM KCl, and 10 mM DTT are sufficient for robust transcription. Stability studies show that Rpo41 loses approximately 30% of its activity during each freeze-thaw cycle, and that the pre-formed elongation complex loses transcriptional activity with a half-life of 7.4±1.5 hr.
Through the use of optical trapping techniques, we have established a method to monitor the transcription of individual Rpo41 molecules in real time. This has allowed us to measure the kinetic rates of nucleotide incorporation by the enzyme: Km = 22±13 µM-1 and vmax = 25±2.5 bp/s. Both of these rates are more similar to those of the main nuclear RNA polymerase in the same organism, RNA Polymerase II (Pol II) than to that of the T7 RNA polymerase, despite the fact that Rpo41 is a single-subunit RNA polymerase with homology to those of the T-odd bacteriophage and no discernable homology to Pol II. Furthermore, like Pol II and the E. coli RNA polymerase, transcription by Rpo41 consists of periods of processive transcription interspersed with periods of pausing. We have also observed retrograde motion of Rpo41 during pauses, termed backtracking, a process that has not been reported in phage-like RNA polymerases.
We have performed single molecule assays of transcription by both Pol II and Rpo41 on templates of differing base pair composition and found that, in general, the characteristics of pausing are attenuated in templates of higher GC content. Specifically, the frequency of pausing is decreased in GC-rich templates, as is the average pause duration. The distribution of pause durations is correspondingly shifted to shorter pauses on GC-rich templates.
We discuss two mechanisms by which template composition may affect pausing: (1) movement of the backtracked transcription bubble is affected by differences in the base stacking energies from the disrupted/created DNA/DNA and RNA/DNA base pairs at the ends of the bubble, and (2) secondary structure of the nascent RNA upstream of the backtracked transcription bubble imposes an energetic barrier to its backward movement. We give in silico evidence that it is the latter mechanism. Incorporation of this secondary structure energy barrier (an "energy penalty") into a model of transcriptional pausing by backtracking allows for statistical fits of the mean pause densities, mean pause durations, and the distribution of pause durations for each enzyme on each template. Furthermore, incorporation of the energy penalty allows for fitting of the pause characteristics for a given enzyme using a single, enzyme specific hopping rate, k0, that is independent of template, and a single, template dependent energy penalty term, ΔGRNA, which is enzyme independent. For Rpo41, we find that k0, the hopping rate of the backtracked enzyme along DNA without RNA secondary structure, is 5.4±1.8 s-1, while it is 2.9±0.3 s-1 for Pol II. Furthermore, the average energy penalty due to the nascent RNA, ΔGRNA, on the AT-rich template used in this study is 0.7±0.1 kT, while it is 0.8±0.1 kT for random DNA and 1.0±0.1 kT for GC-rich DNA.
In order to confirm that it is the secondary structure of the RNA that is the cause of the energy penalty, we performed the same single-molecule transcription assays in the presence of RNase A, an enzyme that digests unprotected RNA in both single-stranded and double-stranded form. The pausing characteristics of all traces on all templates in the presence of RNase A are statistically indistinguishable from those on AT-rich DNA without RNase, indicating that the RNase digested enough of the nascent RNA to disrupt any secondary structure. Protection of the 5' region of the nascent RNA by steric interactions between the polymerase and the RNase prevented full degradation of the RNA, and thus allowed for some backtracking. This strongly supports the new model, presented here, of modulation of transcriptional pausing by secondary structure of the nascent RNA.
In contrast to the detailed and isolated nature of single-molecule transcription, we also performed a synthetic biology project involving Rpo41. The intent of this project was to investigate the plausibility of the creation of a transcriptionally independent mitochondrion, and by extension a minimal cell, by movement of the mitochondrial transcriptional machinery from the nuclear to the mitochondrial genome. Thus we performed in vivo mitochondrial transformation of yeast cells with a synthetic construct containing the gene encoding for Rpo41. We report that we have successfully integrated said synthetic gene into the mitochondrial genome, and have seen its expression to the transcriptional level. Furthermore, we are fairly confident that the full, intact mRNA of the synthetic gene is being created within the mitochondrial matrix.
We have not been able to detect expression of the protein product of the integrated synthetic construct, nor have we been able to isolate a strain that exhibits its expression in the absence of the wild-type, nuclear copy. Because the length of Rpo41 is longer than any other protein synthesized within the mitochondrial organelle, we have begun experiments to determine the maximal polypeptide length able to be translated by the mitochondrial ribosome and associated cofactors.