Nucleosome Positioning around Transcription Start Site Correlates with Gene Expression Only for Active Chromatin State in Drosophila Interphase Chromosomes

We analyzed the whole-genome experimental maps of nucleosomes in Drosophila melanogaster and classified genes by the expression level in S2 cells (RPKM value, reads per kilobase million) as well as the number of tissues in which a gene was expressed (breadth of expression, BoE). Chromatin in 5′-regions of genes we classified on four states according to the hidden Markov model (4HMM). Only the Aquamarine chromatin state we considered as Active, while the rest three states we defined as Non-Active. Surprisingly, about 20/40% of genes with 5′-regions mapped to Active/Non-Active chromatin possessed the minimal/at least modest RPKM and BoE. We found that regardless of RPKM/BoE the genes of Active chromatin possessed the regular nucleosome arrangement in 5′-regions, while genes of Non-Active chromatin did not show respective specificity. Only for genes of Active chromatin the RPKM/BoE positively correlates with the number of nucleosome sites upstream/around TSS and negatively with that downstream TSS. We propose that for genes of Active chromatin, regardless of RPKM value and BoE the nucleosome arrangement in 5′-regions potentiates transcription, while for genes of Non-Active chromatin, the transcription machinery does not require the substantial support from nucleosome arrangement to influence gene expression.