Question

-what is a promoter ???
-what molecule binds it ??

Answer 1

The promoter is where the mRNA message starts. (Note this is different from where the protein translation starts from the mRNA). All sorts of molecules bind - initiation factors included - but the important ones are transcription factors and RNA polymerase. Depending on the species the RNA polymerase might be different but they all do the same job of transcribing the DNA into RNA.

If you have a look at this page then you'll get a reasonable overview: http://en.wikipedia.org/wiki/Promoter_(biology)

Answer 2

In a promoter you will find specific sequences that the transcription factors bind to. These factors have the effect of attracting other factors that can help the RNA polymerase bind. In eukaryotes, the DNA is bound up in nucleosomes (these are loops of DNA wrapped round proteins which help pack the DNA into chromosomes). These nucleosome structures needs to be loosened up to allow RNA polymerase II to bind adn transcription to take place. The other factors I mentioned include chromatin modifying factors like histone acetylases. The post translational modifications of histones are important for regulation of gene expression. I hope this gives you a flavour of the issues around promoters.