Psoralen crosslinking of active and inactive sea urchin histone and rRNA genes

Chromatin structure is highly correlated with the transcriptional activity of specific genes. For example, it has been found that the regularity of nucleosome spacing is compromised when genes are transcribed. The rRNA genes from fungi, plants, and animals give distinctly bimodal distributions of psoralen crosslinking, which has led to the suggestion that these genes might be largely devoid of nucleosomes when transcriptionally active. We investigated the chromatin structure of the multicopy rRNA and histone genes during sea urchin early embryogenesis. The rRNA genes, which are weakly expressed, give a unimodal distribution of weak psoralen crosslinking, in contrast to the situation in all other organisms studied. The early histone genes were more accessible to psoralen crosslinking when active than inactive. The pattern of crosslinking suggests that these polII genes have a homogeneous structure and are still highly protected by nucleosomes when in the active conformation, unlike the situation in polI genes.