Ribosomal gene clusters are uniquely proportioned between open and closed chromatin structures in both tomato leaf cells and exponentially growing suspension cultures.

The accessibility of regulatory molecules to specific DNA sequences and chromatin regions in the nucleus is crucial to gene expression. In this study, we examined the chromatin structure in tomato leaf cells and in exponentially growing tomato cell suspension cultures. The structure of ribosomal chromatin was investigated by micrococcal nuclease and psoralen photocrosslinking. We showed that ribosomal genes in tomato are folded into two distinct types of chromatin: an open chromatin conformation and a closed nucleosomecontaining chromatin. In contrast to previous findings in Friend cells, where half of the ribosomal genes were found to be complexed within an inactive chromatin structure, we demonstrated that the canonical nucleosome-containing chromatin is present in the majority (approximately 80%) of the tomato rRNA-encoding DNA clusters. The minor open chromatin population (approximately 20% of the ribosomal genes) could be detected only after analysis following psoralen crosslinking. The relative amounts of the two ribosomal chromatin structures are similar in stationary and exponentially growing cells. This suggests that the proportions of open and closed chromatin structures present in either stationary or exponentially growing tomato cells are not dependent on the transcriptional process.