U. maydis is a fungal pathogen of corn with two forms: one is yeast-like and nonpathogenic; the other is filamentous and pathogenic. The b locus, with 25 different alleles, regulates this dimorphism: any combination of two different alleles triggers pathogenic development, whereas the presence of identical alleles results in the yeast-like form. We have cloned four b alleles (b1, b2, b3, and b4) and show that the b locus contains a single open reading frame (ORF) of 410 amino acids with a variable N-terminal region and a highly conserved C-terminal region (60% and 93% identity, respectively). Mutational analysis confirms that this ORF is responsible for b activity. The b polypeptides appear to be DNA binding proteins because they contain a motif related to the homeodomain in their constant region. We propose that combinatorial interactions between b polypeptides generate regulatory proteins that determine the developmental program of the fungus.
Nucleosomes were isolated from chromatin of suspension cultured cells of Nicotiana tabacum var. White Burley, which were either habituated or transformed by Agrobacterium tumefaciens, strain T37. Chromatin repeat length in both types of tissue was identical and can be estimated to be 195 ± 10 bp. Using Southern transfer of nucleosomal DNA and hybridization with cloned nick‐translated HindIII fragments of pTi C58 we show that the T‐DNA originating from the Ti‐plasmid of A. tumefaciens is organized in nucleosomes within the chromatin of crown gall tumor cells.
The 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 I1 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 Agrobacten'urn-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 carboxylase/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. Methylatioddemethylation of Alu I-, Fnu4H1-, HaeIII-, Sau3AI-, and Sau 961 sequences in rbcS promoters does not play any role in rbcS gene inactivationlactivation.
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