Binds to the G-box motif (5'-CCACGTGG-3') of the rbcS-1A gene promoter. G-box and G-box-like motifs are cis-acting elements defined in promoters of certain plant genes which are regulated by such diverse stimuli as light-induction or hormone control. DNA-binding heterodimer with GBF2 and GBF3; non DNA-binding homodimer. Belongs to the bZIP family.