Nucleosomes uniquely positioned on high-affinity DNA sequences present a polar 
barrier to transcription by human and yeast RNA polymerase II (Pol II). In one 
transcriptional orientation, these nucleosomes provide a strong, factor- and 
salt-insensitive barrier at the entry into the H3/H4 tetramer that can be 
recapitulated without H2A/H2B dimers. The same nucleosomes transcribed in the 
opposite orientation form a weaker, more diffuse barrier that is largely 
relieved by higher salt, TFIIS, or FACT. Barrier properties are therefore 
dictated by both the local nucleosome structure (influenced by the strength of 
the histone-DNA interactions) and the location of the high-affinity DNA region 
within the nucleosome. Pol II transcribes DNA sequences at the entry into the 
tetramer much less efficiently than the same sequences located distal to the 
nucleosome dyad. Thus, entry into the tetramer by Pol II facilitates further 
transcription, perhaps due to partial unfolding of the tetramer from DNA.