What happens whem rna poloynerase finds a sequence called the

Most cells contain two forms of RNA polymerase. The "core" polymerase is the part that carries out transcription of a gene where the DNA sequence is copied to produce a single-stranded RNA molecule. The core polymerase binds DNA non-specifically as you might expect for a DNA binding protein that has to travel down a large number of different genes. 

When transcription is terminated the core RNA polymerase is released. In order to start a new round of transcription, the core RNA polymerase has to be directed to bind at a promoter, defined as the specific DNA sequence where transcription is initiated. There are specific DNA binding factors that bind to promoters and to RNA polymerase. That's how they direct RNA polymerase to the place where transcription has to start. These factors bind first to core polymerase forming the second form of RNA polymerase called the holoenzyme.

The binding parameters of the E. coli core polymerase and the holoenzyme have been studied in detail. In E. coli cells there are several different versions of holoenzyme. Each one contains a different initiation factor that binds to a different series of promoters. The most common initiation factor is called σ70 (sigma-70) and it binds to most of the promoters in the cell.

The steps in transcription initiation are shown in the figure. First, holoenzyme consisting of core polymerase + σ70, binds non-specifically to any stretch of DNA. It then moves along the DNA in a one-dimensional search until it finds a promoter sequence. This is followed by a local unwinding of the DNA and synthesis of a short piece of DNA.