  An important step in transcription of a DNA base sequence to a protein is the
initiation from the exact starting point, called promoter region. We propose a
physical mechanism for identification of the promoter region, which relies on a
new classification of DNAs into two types, Type-I and Type-II, like
superconductors, depending on the sign of the energy of the interface
separating the zipped and the unzipped phases. This is determined by the
energies of helical ordering and stretching over two independent length scales.
The negative interfacial energy in Type II DNA leads to domains of helically
ordered state separated by defect regions, or blobs, enclosed by the
interfaces. The defect blobs, pinned by non-coding promoter regions, would be
physically distinct from all other types of bubbles. We also show that the
order of the melting transition under a force is different for Type I and Type
II.
