Controlling Heterogeneity and Increasing Titer from Riboswitch-Regulated Bacillus subtilis Spores for Time-Delayed Protein Expression Applications

Abstract

Sporulated cells have potential as time-delayed expression chassis of proteins for applications such as on-demand biologics production, whole cell biosensors, or oral vaccines. However, the desired attributes of high expression rates and low product variances are difficult to maintain from germinated spores. In this work we study the effect of an integrating vs. theta replicating plasmid in a wild-type Bacillus subtilis and two PolY mutants. The cells were engineered to produce a fluorescent reporter protein (RFP) under the control of a riboswitch activated by theophylline. This allowed for greater sensitivity to point mutations. The fluorescence and cell growth curves were fit with a custom kinetic model and a peak kinetic rate (LKPmax) was extracted for each clonal population (n = 30 for all cell, vector, and growth combinations). Plasmid based expression yields higher (8.7x) expression rates due to an increased copy number of the expression cassette (10x over integrated). The variance of LKPmax values increased 2.07x after sporulation for the wild type strain. This increase in variance from sporulation is very similar to what is observed with UV exposure. This effect can be partially mitigated by the use of PolY knockouts observed in suspended cell growths and adherent biofilms.