tokens: By using a nucleosome -assembly system derived from Drosophila embryos , we find that the packaging of DNA into chromatin in vitro strongly represses HIV-1 transcription and that repression can be counteracted efficiently by preincubation of the DNA with LEF-1 ( or LEF-1 and Ets-1 ) supplemented with fractions containing the promoter-binding protein , Sp1 .