GAGA transcription factor (GAF) is an essential protein in Drosophila,
important for the transcriptional regulation of numerous genes. GAF b
inds to GA repeats in the promoters of these genes via a DNA-binding d
omain containing a single zinc finger. While GAF binding sites are typ
ically composed of 3.5 GA repeats, the Drosophila hsp70 gene contains
much smaller elements, some of which are as little as three bases (GAG
) in length. Interestingly, the binding of GAF to more distant trinucl
eotide elements is relatively strong and not appreciably affected by t
he removal of larger GA arrays in the promoter, Moreover, a simple syn
thetic GAG sequence is sufficient to bind GAF in vitro. Here we direct
ly compare the affinity of GAF for different sequence elements by immu
noprecipitation and gel mobility shin analysis, Furthermore, our measu
res of the concentration of GAF in vivo indicate that it is a highly a
bundant nuclear protein, prevalent enough to occupy a sizable fraction
of correspondingly abundant trinucleotide sites.