Cold-Inducible Promoters for Heterologous Protein Expression

Rapid transfer of exponentially growing E. coli cultures from physiological to low temperatures (10–15#X00B0;C) has profound consequences on cell physiology: membrane fluidity decreases, which interferes with transport and secretion, the secondary structures of nucleic acids are stabilized, which affect the efficiencies of mRNA transcription/translation and DNA replication, and free ribosomal sub-units and 70S particles accumulate at the expense of polysomes, negatively impacting translation of most cellular mRNAs (1 –3 ). It is therefore not surprising that cell growth and the synthesis of the vast majority of cellular proteins abruptly stop upon sudden temperature downshift (4 ). However, this lag phase is only transient, and growth resumes with reduced rates after 2-4 h incubation at low temperatures, depending on the genetic background (4 ,5 ). Such remarkable ability to survive drastic changes in environmental conditions is not atypical for E. coli , which has evolved multiple, often synergistic, adaptive strategies to handle stress. In the case of cold shock, the need for restoring transcription and translation is handled by an immediate increase in the synthesis of about 16 cold shock proteins (Csps) (4 ), while the cell solves the problem of membrane fluidity by raising the concentration of unsaturated fatty acids that are incorporated into membrane phospholipids (6 ). Interestingly, translation of the alternative sigma factor σ(suS), a global regulator of gene expression in E. coli , has been reported to increase at 20�C (7) suggesting that RpoS-dependent gene products may also play a role in cellular adaptation to mild—but probably not severe—cold shock.