Final answer:
The GC box is a promoter sequence found upstream of the TATA box in eukaryotes that helps increase transcription efficiency; its consensus sequence is GGCG.
Step-by-step explanation:
The GC box is a nonessential eukaryotic promoter sequence that plays a role in increasing the efficiency of transcription. It is generally located upstream of the TATA box within the promoter region. The consensus sequence of the GC box is GGCG, and it may be present several times within a promoter to bind cellular factors. Note that while it is a nonessential component, it supports transcriptional activity in genes where it is present.