Constitutive Expression Vectors: PGK

The promoter of the yeast gene encoding the glycolytic enzyme phosphoglycerate kinase (PGK) has been used to construct vectors for expression of heterologous proteins in budding yeast (Saccharomyces cerevisiae ) (1 –4 ). This promoter is one of the most efficient yeast promoters and is used when a high level of constitutive gene expression is required. Most PGK -based vectors are constructed using high copy number plasmids, to maximize expression levels of heterologous sequences. When present on such a plasmid, the promoter can drive production of PGK protein up to 30–40% of total cell protein, although much lower levels are obtained when heterologous sequences are expressed (5 ,6 ). When the human IFNα2 gene was expressed using the high copy number plasmids pMA230-1 and pMA301-1, interferon was only produced to the level of 1–3% of total cell protein (6 ). Similar low yields have also been described for several other proteins expressed using PGK systems (7 ,8 ). The reasons for these less than maximum levels of expression are not completely understood, although several suggestions have been made. These range through positive feedback by the PGK protein, RNA stability effects, and the presence of a transcriptional enhancer within the PGK coding region (9 ,10 ). Despite this underperformance, PGK promoter systems are useful because they allow relatively high level gene expression under very simple conditions.