Internal promoter characterization and expression of the Deinococcus radiodurans pprI-folP gene cluster

PprI is a general gene switch responsible for the extraordinary radioresistance of Deinococcus radiodurans. From NCBI DNA sequence analysis, it was predicted that the translation start codon of the downstream folP (DR0168) gene overlaps the pprI (DR0167) stop codon, suggesting that these genes may form an operon. In this study, we show that a mutant containing an inserted sequence in folP does not grow unless folate is added to the medium, but is not affected in extreme radioresistance, whereas a pprI disruptant strain could grow in the absence of folate. It was found that expression of a pprI-lacZ fusion is constitutive and unaltered following ionizing radiation as is the production of the PprI protein. PprI protein is not expressed if its promoter is deleted and the transcription from the entire pprI promoter is essential for radioresistance of D. radiodurans. However, the deletion of pprI promoter has no effect on the expression of the folP-lacZ fusion. Primer extension analysis of the folP promoter region shows that folP is transcribed from its own promoter located within the pprI structural gene. All these results do support neither the existence of a pprI-folP operon nor a regulatory role of FolP in pprI expression.