MRG15 is a novel chromodomain protein that is a member of a family of genes 
related to MORF4. MORF4 (mortality factor on chromosome 4) induces senescence in 
a subset of human tumor cell lines. Our previous results indicated that MRG15 
(MORF-related gene on chromosome 15) could derepress the B-myb promoter by 
association with Rb. In this study, sucrose gradient analysis demonstrated that 
MRG15 was present in two distinct nuclear protein complexes, MAF1 
(MRG15-associated factor 1) and MAF2. Rb was associated with MRG15 and PAM14 (a 
novel coil-coil protein) in MAF1, and a histone acetyl transferase, hMOF, was an 
MRG15 partner in MAF2. Analysis of deletion mutants of MRG15 indicated that the 
leucine zipper at the C-terminal region of MRG15 was important for the protein 
associations in MAF1 and that the N-terminal chromodomain was required for the 
assembly of the MAF2 protein complex. Consistent with these data was the fact 
that a histone acetyltransferase activity associated with MRG15 was lost when 
the chromodomain was deleted and that both mutant MRG15 proteins failed to 
activate the B-myb promoter. The various mechanisms by which MRG15 could 
activate gene transcription are discussed.