 The former, that is the core element, is located around the start site of transcription and the later, that is the UCE element, between 100 and 150 base pairs upstream in humans. In addition to Polymerase 1, initiation requires two other factors which are called SL1 and UBF. SL1 comprises TBP and 3 TAFs specific for Polymerase 1 transcription. This complex binds to the core element in the promoter. So, here you can see this is the core element and this one is UCE. This is the promoter and here it is RNA polymerase 1 and this whole is SL1 which contains many TAFs, TAFs, TBP and some other factors. So, this is the transcription machinery for Polymerase 1. SL1 binds DNA only in the presence of UBF. This factor binds to UCE bringing in SL1 and stimulating transcription from the core promoter by recruiting Polymerase 1. So, only then transcription starts when all these bind together. Polymerase 3 promoters come in various forms and the vast majority have the unusual feature of being located downstream from the transcription start site. So, all the promoters are present upstream from the transcription start site but the promoters for Polymerase 3, they are present downstream from the start site of transcription. So, they come within the coding region of the gene. Some Polymerase 3 promoters for example those for the tRNA genes consist of two regions called box A and box B which are separated by a short element. So, this is box A, box B and in between these two is a short element. So, this is promoter for tRNA gene. Others contain box A and box C for example the 5S rRNA gene and still others contain a Tata element like those of Polymerase 2. Transcription by Polymerase 3 also requires transcription factors in addition to Polymerase. So, like Polymerase 1 and Polymerase 2 this Polymerase 3 also requires transcription factors. In this case the factors are called TF3B and TF3C for the tRNA genes and those TF3A for 5S rRNA gene. So, this is the transcription machinery. So, this is a tRNA gene and this is box A, box B and this orange one is TF3C. This is Polymerase 3, this is TBP and these are two which are the subunits of TF2B. So, in case of 5S rRNA gene the promoter contains box A and box C while in some other cases Tata box and PSE box are also present. The TF3C complex binds to the promoter region. This complex recruits TF3B to the DNA just upstream of the start site where it in turn recruits Polymerase 3 to the start site of transcription. So, first this TFC complex binds to the promoter then this TF3C recruits TF3B and then this TF3B recruits Polymerase 3 and then transcription starts. The enzyme then initiates presumably displacing TF3C from the DNA template at it as it goes. So, this TF3C this is displaced from the DNA strand and then transcription starts. Polymerase 3 also uses TBP like other polymerases in this case that ubiquitous factor is found within the TF3B complex. So, this TBP is present within the TF3B complex in case of Polymerase 3 transcription.