Heterogeneity in Bacillus subtilis : growth phase dependent activity and noise

Abstract

Stochastic noise is a naturally occurring phenomena in all chemical reactions. The processes involved in gene expression are subject to the stochastic, random nature of chemical interactions. Single cell measurements of fluorescent activity have been used to deconstruct the sources of gene expression noise. These systems rely on the use of inducible gene expression from negatively regulated promoters. Gene expression noise is quantified by the fluorescent activity of the reporter. Single time-point assays have defined the contribution of transcription and translation as sources of gene expression noise. This thesis investigated gene expression noise in Bacillus subtilis. It extends the single time-point assays and places noise within the context of the bacterial growth curve. Two main findings were concluded from the data. Firstly, there is growth phase dependent fluorescent activity, due to the accumulation of a stable fluorescent protein. Secondly, high noise levels in gene expression are a transcription dependent feature of cells in stationary phase. Investigating the synthetic gene circuits responsible for these phenotypes highlight the importance of fully characterising the system. Differences in transcription, translation and fluorescent activity were observed in response to the architecture of the gene circuits