Comparison of E. coli based self-inducible expression systems containing different human heat shock proteins.

IPTG-inducible promoter is popularly used for the expression of recombinant proteins. However, it is not suitable at the industrial scale due to the high cost and toxicity on the producing cells. Recently, a Self-Inducible Expression (SILEX) system has developed to bypass such problems using Hsp70 as an autoinducer. Herein, the effect of other heat shock proteins on the autoinduction of green fluorescent protein (EGFP), romiplostim, and interleukin-2 was investigated. For quantitative measurements, EGFP expression was monitored after double-transformation of pET28a-EGFP and pET21a-(Hsp27/Hsp40/Hsp70) plasmids into E. coli using fluorimetry. Moreover, the expression level, bacterial growth curve, and plasmid and expression stability were compared to an IPTG- inducible system using EGFP. Statistical analysis revealed a significant difference in EGFP expression between autoinducible and IPTG-inducible systems. The expression level was higher in Hsp27 system than Hsp70/Hsp40 systems. However, the highest amount of expression was observed for the inducible system. IPTG-inducible and Hsp70 systems showed more lag-time in the bacterial growth curve than Hsp27/Hsp40 systems. A relatively stable EGFP expression was observed in SILEX systems after several freeze-thaw cycles within 90 days, while, IPTG-inducible system showed a decreasing trend compared to the newly transformed bacteria. Moreover, the inducible system showed more variation in the EGFP expression among different clones than clones obtained by SILEX systems. All designed SILEX systems successfully self-induced the expression of protein models. In conclusion, Hsp27 system could be considered as a suitable autoinducible system for protein expression due to less metabolic burden, lower variation in the expression level, suitable plasmid and expression stability, and a higher expression level.