what is the purpose of a positive transcription factor

Question

a tran factor is required for RNA polymerase to initiate transcription at specific promoter, but is not itself part of the enzym. 

Answer 1

To add to Noel's answer, positive transcription factor or activator bind to its activation site in the promoter region and modulate the DNA-protein or chromatin configuration in a manner that facilitates RNA polymerase binding at specific transcription start site for the onset of transcription.

 

Some transcription factors have dual function: they work as activator and inhibitors of transcription on different set of genes. Onset and termination of transcription require number of factors as well as chromatin modulation. Therefore, same transcription factor accomplishes dual function based on cell and tissue type and the available  associated factors.

 

Hope this helps as well. 

Answer 2

Transcription factors can bind directly to DNA. They do so in special regions such as the promoter region or regulatory sequences. Positive transcription factors promote transcription. They are needed in order for RNA polymerase to begin transcription.