Measurement of the copy number of the master quorum-sensing regulator of a bacterial cell

TL;DR

This study quantifies the LuxR protein copy number in Vibrio harveyi using fluctuation analysis during cell division, providing insights into quorum sensing regulation and demonstrating a broadly applicable in vivo measurement method.

Contribution

The paper introduces a novel fluctuation-based method to measure protein copy number in vivo, specifically applied to LuxR in Vibrio harveyi, revealing density-dependent variations.

Findings

LuxR copy number ranges from 80-135 dimers at low density to 575 at high density

The fluorescence distribution narrows with increasing LuxR population

The method is broadly applicable for in vivo protein quantification

Abstract

Quorum sensing is the mechanism by which bacteria communicate and synchronize group behaviors. Quantitative information on parameters such as the copy number of particular quorum-sensing proteins should contribute strongly to understanding how the quorum-sensing network functions. Here we show that the copy number of the master regulator protein LuxR in Vibrio harveyi, can be determined in vivo by exploiting small-number fluctuations of the protein distribution when cells undergo division. When a cell divides, both its volume and LuxR protein copy number N are partitioned with slight asymmetries. We have measured the distribution functions describing the partitioning of the protein fluorescence and the cell volume. The fluorescence distribution is found to narrow systematically as the LuxR population increases while the volume partitioning is unchanged. Analyzing these changes statistically, we have determined that N = 80-135 dimers at low cell density and 575 dimers at high cell density. In addition, we have measured the static distribution of LuxR over a large (3,000) clonal population. Combining the static and time-lapse experiments, we determine the magnitude of the Fano factor of the distribution. This technique has broad applicability as a general, in vivo technique for measuring protein copy number and burst size.