The effect of heating rate on Escherichia coli metabolism, physiological stress, transcriptional response, and production of temperature‐induced recombinant protein: A scale‐down study

At the laboratory scale, sudden step increases from 30 to 42°C can be readily accomplished when expressing heterologous proteins in heat‐inducible systems. However, for large scale‐cultures only slow ramp‐type increases in temperature are possible due to heat transfer limitations, where the heating rate decreases as the scale increases. In this work, the transcriptional and metabolic responses of a recombinant Escherichia coli strain to temperature‐induced synthesis of pre‐proinsulin in high cell density cultures were examined at different heating rates. Heating rates of 6, 1.7, 0.8, and 0.4°C/min were tested in a scale‐down approach to mimic fermentors of 0.1, 5, 20, and 100 m3, respectively. The highest yield and concentration of recombinant protein was obtained for the slowest heating rate. As the heating rate increased, the yield and maximum recombinant protein concentration decreased, whereas a larger fraction of carbon skeletons was lost as acetate, lactate, and formate. Compared to 30°C, the mRNA levels of selected heat‐shock genes at 38 and 42°C, as quantified by qRT‐PCR, increased between 2‐ to over 42‐fold when cultures were induced at 6, 1.7, and 0.8°C/min, but no increase was observed at 0.4°C/min. Only small increases (between 1.5‐ and 4‐fold) in the expression of the stress genes spoT and relA were observed at 42°C for cultures induced at 1.7 and 6°C/min, suggesting that cells subjected to slow temperature increases can adapt to stress. mRNA levels of genes from the transcription–translation machinery (tufB, rpoA, and tig) decreased between 40% and 80% at 6, 1.7 and 0.8°C/min, whereas a transient increase occurred for 0.4°C/min at 42°C. mRNA levels of the gene coding for pre‐proinsulin showed a similar profile to transcripts of heat‐shock genes, reflecting a probable analogous induction mechanism. Altogether, the results obtained indicate that slow heating rates, such as those likely to occur in conventional large‐scale fermentors, favored heterologous protein synthesis by the thermo‐inducible expression system used in this report. Knowledge of the effect of heating rate on bacterial physiology and product formation is useful for the rational design of scale‐down and scale‐up strategies and optimum recombinant protein induction schemes. Biotechnol. Bioeng. 2009;102: 468–482. © 2008 Wiley Periodicals, Inc.