Which eukaryotic promoter element is essential and works by binding transcription factors?

1. The CAAT box has the sequence GGCCAATCT and sits at approximately -80 in the mouse thymidine kinase promoter. 2. It is an essential promoter sequence and its role is to bind transcription factors. 3. The GC-rich box and the octamer box are nonessential, so they can be missing and transcription still proceeds. 4. Those two only raise the efficiency of initiation and may appear several times in one promoter. 5. The poly-A signal acts on the transcript during processing rather than on the promoter, so it is not a promoter element at all. 6. Only the CAAT box satisfies both conditions in the question. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 15 "Genes and Proteins", section 15.3 Eukaryotic Transcription_