The A-myb gene belongs to the family of the c-myb proto-oncogene. We r
eport here the cloning from a B lymphocyte cDNA library of the previou
sly missing 3' half of the human A-myb cDNA, thus closing the previous
ly still incomplete open reading frame. Analysis of the homologies bet
ween the different myb proteins reveals four domains of high conservat
ion. We show, using a polyclonal rabbit antibody, that the 90 kd human
A-myb protein is nuclear and that it activates transcription from the
KHK-CAT reporter 6-10 times more strongly than c-myb in NIH3T3 cells.
The transactivating function of A-myb depends on the presence of the
myb binding site in the reporter, and on both the DNA binding and acid
ic domains of the A-myb protein. The bacterially expressed protein pro
tects the myb binding sites of the reporter in footprint experiments.
Binding of the A-myb protein is shown in gel retardation assays to be
specific for the classical c-myb recognition sequence PyAAC(G)/(T)G. I
n addition, like c-myb, A-myb binds more strongly to the MIM-A synthet
ic oligonucleotide that carries the TAACGG sequence than to the MBS-I
oligonucleotide containing TAAGTG. Finally, DNA binding activity is de
monstrated to require the N-terminal portion of the protein containing
the three tandem repeats of amino acids conserved in all myb proteins
. We have thus shown that the A-myb protein is a strong activator of t
ranscription and that this activity depends on both the DNA-binding an
d acidic domains.