Variants of Concern (VOC) of SARS-CoV-2, including Alpha, Beta, Gamma, Delta, and Omicron threaten to prolong the pandemic leading to more global morbidity and mortality. Genome sequencing is the mainstay of tracking the development and evolution of the virus, but is costly, slow, and not easily accessible. Multiplex qRT-PCR assays for SARS-CoV-2 were developed, which identify all VOC as well as other mutations of interest in the viral genome, nine mutations in total, using single nucleotide discriminating molecular beacons. The presented variant molecular beacon assays showed a limit of detection of fifty copies of viral RNA, with 100% specificity. Twenty-six SARS-CoV-2 positive patient samples were blinded and tested using a two-tube assay. When testing patient samples, the assay was in full agreement with results from deep sequencing with a sensitivity and specificity of 100% (26/26). We have used our design methodology to rapidly design an assay which detects the new Omicron variant. This Omicron assay was used to accurately identify this variant in 17 of 33 additional patient samples. These qRT-PCR assays identify all currently circulating VOC of SARS-CoV-2 as well as other important mutations in the Spike protein coding sequence. These assays can be easily implemented on broadly available five-color thermal cyclers and will help track the spread of these variants.

Many eukaryotic genes are expressed in randomly initiated bursts that are punctuated by periods of quiescence. Here, we show that the intermittent access of the promoters to transcription factors through relatively impervious chromatin contributes to this “noisy” transcription. We tethered a nuclease-deficient Cas9 fused to a histone acetyl transferase at the promoters of two endogenous genes in HeLa cells. An assay for transposase-accessible chromatin using sequencing showed that the activity of the histone acetyl transferase altered the chromatin architecture locally without introducing global changes in the nucleus and rendered the targeted promoters constitutively accessible. We measured the gene expression variability from the gene loci by performing single-molecule fluorescence in situ hybridization against mature messenger RNAs (mRNAs) and by imaging nascent mRNA molecules present at active gene loci in single cells. Because of the increased accessibility of the promoter to transcription factors, the transcription from two genes became less noisy, even when the average levels of expression did not change. In addition to providing evidence for chromatin accessibility as a determinant of the noise in gene expression, our study offers a mechanism for controlling gene expression noise which is otherwise unavoidable.

Modulating fluorescent protein emission holds great potential for increasing readout sensitivity for applications in biological imaging and detection. Here, we identify and engineer optically modulated yellow fluorescent proteins (EYFP, originally 10C, but renamed EYFP later, and mVenus) to yield new emitters with distinct modulation profiles and unique, optically gated, delayed fluorescence. The parent YFPs are individually modulatable through secondary illumination, depopulating a long-lived dark state to dynamically increase fluorescence. A single point mutation introduced near the chromophore in each of these YFPs provides access to a second, even longer-lived modulatable dark state, while a different double mutant renders EYFP unmodulatable. The naturally occurring dark state in the parent YFPs yields strong fluorescence modulation upon long-wavelength-induced dark state depopulation, allowing selective detection at the frequency at which the long wavelength secondary laser is intensity modulated. Distinct from photoswitches, however, this near IR secondary coexcitation repumps the emissive S1 level from the long-lived triplet state, resulting in optically activated delayed fluorescence (OADF). This OADF results from secondary laser-induced, reverse intersystem crossing (RISC), producing additional nanosecond-lived, visible fluorescence that is delayed by many microseconds after the primary excitation has turned off. Mutation of the parent chromophore environment opens an additional modulation pathway that avoids the OADF-producing triplet state, resulting in a second, much longer-lived, modulatable dark state. These Optically Modulated and Optically Activated Delayed Fluorescent Proteins (OMFPs and OADFPs) are thus excellent for background- and reference-free, high sensitivity cellular imaging, but time-gated OADF offers a second modality for true background-free detection. Our combined structural and spectroscopic data not only gives additional mechanistic details for designing optically modulated fluorescent proteins but also provides the opportunity to distinguish similarly emitting OMFPs through OADF and through their unique modulation spectra.

Variants of Concern (VOC) of SARS-CoV-2, including Alpha, Beta, Gamma, and Delta, threaten to prolong the pandemic leading to more global morbidity and mortality. Genome sequencing is the mainstay of tracking the development and evolution of the virus, but is costly, slow, and not easily accessible. A multiplex qRT-PCR assay for SARS-CoV-2 was developed, which identifies all VOC as well as other mutations of interest in the viral genome, eight mutations total, using single nucleotide discriminating molecular beacons in a two-tube assay. The sensitivity and specificity of the assay was tested using in vitro-transcribed targets. Twenty-six SARS-CoV-2 positive patient samples were blinded, then tested using this assay and compared with deep sequencing results. The presented variant molecular beacon assay showed high accuracy when testing in vitro-transcribed targets, down to a limit of detection of five copies of the viral RNA, with 100% specificity. When testing patient samples, the assay was in full agreement with results from deep sequencing with a sensitivity and specificity of 100% (26/26). We have developed a qRT-PCR assay for the identification of currently circulating VOC of SARS-CoV-2 as well as other important mutations in its Spike protein coding sequence. This assay can be easily implemented on broadly available five-color thermal cyclers and will help track the spread of these variants.

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