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Background  Repressible promoters are a useful tool for down-regulating the expression of genes, especially those that affect cell viability, in order to study cell physiology. They are also popular in biotechnological processes, like heterologous protein production.    Results  Here we present five novel repressible  Pichia pastoris  promoters of different strength: P  SER1  , P  MET3  , P  THR1  , P  PIS1   and P  THI11  . eGFP was expressed under the control of each of these promoters and its fluorescence could be successfully decreased in liquid culture by adding different supplements. We also expressed the essential genes with different native promoter strength,  ERO1  and  PDI1 , under the control of two of the novel promoters. In our experiments, a clear down-regulation of both repressible promoters on transcriptional level could be achieved. Compared to the transcript levels of these two genes when expressed under the control of their native promoters, only  ERO1  was significantly down-regulated.    Conclusion  Our results show that all of the novel promoters can be used for repression of genes in liquid culture. We also came to the conclusion that the choice of the repressible promoter is of particular importance. For a successful repression experiment it is crucial that the native promoter of a gene and the repressible promoter in its non-repressed state are of similar strength.

Repressible promoters – A novel tool to generate conditional mutants in Pichia pastoris