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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.