Any process involved in the melting of the DNA hybrid of the core promoter region within the transcriptional closed complex of an RNA polymerase preinitiation complex (PIC) to produce an open complex where the DNA duplex around the transcription initiation site is unwound to form the transcription bubble. [ http://www.ncbi.nlm.nih.gov/pubmed/18280161 http://www.ncbi.nlm.nih.gov/pubmed/15020047 ]