Chromatin structure and plant gene expression

Abstract

AbstractThe nuclear DNA of eukaryotic organisms is associated with a variety of proteins, which together make up what is called “chromatin.” Chromatin serves to package all genes into higher‐order structures such as nucleosomes, solenoids, and loop domains. Tight packing of a particular gene and its regulatory sequences does not allow the approach of RNA I or II polymerase proteins. Before or during the activation of such an inactive gene its chromatin has to adopt a relaxed, more “open” configuration. This altered chromatin can be probed by its higher sensitivity toward nucleases, such as DNAse I or S1 nuclease, and the appearance of DNAse I‐hypersensitive sites. These sites may constitutively be present or may be induced, and they can be mapped to specific DNA sequence motifs. In many cases, such sites are delimited by non‐B‐DNA, notably Z‐DNA, which in turn may form part of enhancer elements. The Z‐DNA configuration may be induced or maintained by methylation of cytosyl residues within underlying sequences.Two plant gene model systems have been selected to probe their chromatin structure. Constitutively expressed T‐DNA genes of Agrobacterium induced tobacco crown gall tumor cells have been shown to be organized in canonical nucleosomes, to be more sensitive to DNAse I than the bulk of host chromatin, and to contain a series of six constitutive DNAse I‐hypersensitive sites. Inducible ribulose‐1,5‐bisphosphate carboxyl‐ase/oxygenase small subunit (rbcS) genes of pea are rearranged into a nuclease‐sensitive format upon activation by light, especially in their promoter region. The rbcS promoter harbors a series of five constitutive DNAse I‐hypersensitive sites and one light‐inducible site, which is surrounded by potential regulatory sequences (enhancer cores, inverted repeats). The 3′ region of rbcS genes also contains constitutive sites. Methylation/demeth‐ylation of Alu I‐, Fnu4H1‐, HaeIII‐, Sau3AI‐, and Sau 96I sequences in rbcS promoters does not play any role in rbcS gene inactivation/activation.