Fig. 4.

Bidirectional promoters in intron-retaining genes. a Gene orientation scheme with arrow heads at the 3′ end. b Histograms of binned intergenic distances between intron-retaining genes (right). The intergenic distance is determined as distance (in kb) between the transcription start sites of two genes (–/+; HH) or end of transcripts (+/–; TT), when on opposite strands, and between end of transcript and transcription start site, when both genes are on the same strand (+/+ or –/–; TH). The percentages indicated in each plot refer to the fraction of gene pairs with an intergenic distance of ≤ 1 kb