Although the figure is drawn as a straight line, the binding of transcription factors to each other probably draws the DNA of the promoter into a loop. Many different genes and many different types of cells share the same transcription factors - not only those that bind at the basal promoter but even some of those that bind upstream. What turns on a particular gene in a particular cell is probably the unique combination of promoter sites and the transcription factors that are chosen. The rows of lock boxes in a bank provide a useful analogy. Link to a discussion of how the DNA sequence of promoter sites can be determined.
Transcription factors represent only a small fraction of the proteins in a cell.
Link to a discussion of how they can nonetheless be isolated and purified. Hormones exert many of their effects by forming transcription factors. The complexes of hormones with their receptor represent one class of transcription factor. Link to a discussion of these. Embryonic development requires the coordinated production and distribution of transcription factors.