Single-Cell Biophysics: Measurement, Modulation, and Modeling

Single-Cell Biophysics: Measurement, Modulation, and Modeling

Sunday Speaker Abstracts

Spatial Organization of Transcription in E. coli via Superresolution Fluorescence

Microscopy

Xiaoli Weng

, Christopher H. Bohrer

, Arvin C. Lagda

, Jie Xiao

Johns Hopkins School of Medicine, Baltimore, MD, USA,

Icahn School of Medicine at Mount

Sinai, New York, NY, USA.

Transcription, the process of converting genetic information stored in DNA to RNA, lies at the

heart of gene expression. Transcription has been studied extensively in-vitro to probe its

mechanistic detail, however, these conditions differ from the complex environment inside a

living cell. Spatial distributions of molecular components have recently been shown to be an

important facet of gene regulation in prokaryotic systems. We investigated the spatial

distributions of various molecular components of transcription in

E. coli

and their physical

correlation with each other, to gain insight into the regulation of gene expression at the global,

cellular level. Using superresolution fluorescence microscopy, we found that RNA Polymerase

(RNAP) forms distinct clusters under fast growth that are largely retained in cells under different

global transcription perturbations. RNAP clusters are the most homogenously distributed in cells

without active transcription via rifampicin treatment. Additionally, we used multi-color

superresolution imaging to correlate the spatial localization of RNAP clusters with DNA sites,

nascent rRNA and elongation factor NusA to further elucidate the underlying make-up of RNAP

clusters. Our results show that RNAP clusters are highly co-localized with NusA, thus are likely

composed of elongation complexes. Interestingly, while RNAP clusters have a certain level of

colocalization with nascent rRNA, and rrn DNA sites in fast growing cells, RNAP clusters are

retained under conditions where rRNA synthesis is reduced. This points to the independence of

formation of RNAP clusters from active rRNA synthesis. These results suggest a high level of

heterogeneity both in the spatial organization and functional role of RNAP clusters in

E. coli

Old and New News about Single-Photon Sensitivity in Human Vision

Philip Nelson

University of Pennsylvania, Philadelphia, PA, USA.

One often hears that human vision is “sensitive to single photons,” when in fact the faintest flash

of light that can reliably be reported by human subjects is closer to 100 photons. Nevertheless,

there is a sense in which the familiar claim is true. Experiments conducted long after the seminal

work of Hecht, Shlaer, and Pirenne now admit a more precise, and in some ways even more

remarkable, conclusion to be drawn about our visual apparatus. A simple model that incorporates

both old news (response of single rod cells) and newer news (loss at the first synapse) can

account in detail for both old and new psychophysical data.

[Work supported in part by US NSF grant PHY--1601894.]