Modification of histones, DNA-binding proteins found in chromatin, by addition of acetyl groups occurs to a greater degree when the histones are associated with transcriptionally active DNA. A breakthrough in understanding how this acetylation is mediated was the discovery that various transcriptional co-activator proteins have intrinsic histone acetyltransferase activity (for example, Gcn5p, PCAF, TAF(II)250 and p300/CBP. These acetyltransferases also modify certain transcription factors (TFIIEbeta, TFIIF, EKLF and p53). GATA-1 is an important transcription factor in the haematopoietic lineage and is essential for terminal differentiation of erythrocytes and megakaryocytes. It is associated in vivo with the acetyltransferase p300/CBP. Here we report that GATA-1 is acetylated in vitro by p300. This significantly increases the amount of GATA-1 bound to DNA and alters the mobility of GATA-1-DNA complexes, suggestive of a conformational change in GATA-1. GATA-1 is also acetylated in vivo and acetylation directly stimulates GATA-1-dependent transcription. Mutagenesis of important acetylated residues shows that there is a relationship between the acetylation and in vivo function of GATA-1. We propose that acetylation of transcription factors can alter interactions between these factors and DNA and among different transcription factors, and is an integral part of transcription and differentiation processes.

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Repression of transcription from densely methylated genes can be mediated by the methyl‐CpG binding protein MeCP‐1 (Boyes and Bird, 1991). Here we have investigated the effect of methylation on genes with a low density of methyl‐CpG. We found that sparse methylation could repress transfected genes completely, but the inhibition was fully overcome by the presence in cis of an SV40 enhancer. Densely methylated genes, however, could not be reactivated by the enhancer. In vitro studies showed that the sparsely methylated genes bound weakly to MeCP‐1 and that binding interfered with transcription. In the absence of available MeCP‐1, methylation had minimal effects on transcription. From these and other results we propose that sparsely methylated genes form an unstable complex with MeCP‐1 which prevents transcription when the promoter is weak. This complex can be disrupted by a strong promoter, thereby allowing the methylated gene to be transcribed.