We have characterized five genes encoding condensin components in Saccharomyces cerevisiae. All genes are essential for cell viability and encode proteins that form a complex in vivo. We characterized new mutant alleles of the genes encoding the core subunits of this complex, smc2-8 and smc4-1. Both SMC2 and SMC4 are essential for chromosome transmission in anaphase. Mutations in these genes cause defects in establishing condensation of unique (chromosome VIII arm) and repetitive (rDNA) regions of the genome but do not impair sister chromatid cohesion. In vivo localization of Smc4p fused to green fluorescent protein showed that, unexpectedly, in S. cerevisiae the condensin complex concentrates in the rDNA region at the G2/M phase of the cell cycle. rDNA segregation in mitosis is delayed and/or stalled in smc2 and smc4 mutants, compared with separation of pericentromeric and distal arm regions. Mitotic transmission of chromosome III carrying the rDNA translocation is impaired in smc2 and smc4 mutants. Thus, the condensin complex in S. cerevisiae has a specialized function in mitotic segregation of the rDNA locus. Chromatin immunoprecipitation (ChIP) analysis revealed that condensin is physically associated with rDNA in vivo. Thus, the rDNA array is the first identified set of DNA sequences specifically bound by condensin in vivo. The biological role of higher-order chromosome structure in S. cerevisiae is discussed.

We have determined the relationship between overall nuclear architecture, chromosome territories, and transcription sites within the nucleus, using three-dimensional confocal microscopy of well preserved tissue sections of wheat roots. Chromosome territories were visualized by GISH using rye genomic probe in wheat/rye translocation and addition lines. The chromosomes appeared as elongated regions and showed a clear centromere–telomere polarization, with the two visualized chromosomes lying approximately parallel to one another across the nucleus. Labeling with probes to telomeres and centromeres confirmed a striking Rabl configuration in all cells, with a clear clustering of the centromeres, and cell files often maintained a common polarity through several division cycles. Transcription sites were detected by BrUTP incorporation in unfixed tissue sections and revealed a pattern of numerous foci uniformly distributed throughout the nucleoplasm, as well as more intensely labeled foci in the nucleoli. It has been suggested that the gene-rich regions in wheat chromosomes are clustered towards the telomeres. However, we found no indication of a difference in concentration of transcription sites between telomere and centromere poles of the nucleus. Neither could we detect any evidence that the transcription sites were preferentially localized with respect to the chromosome territorial boundaries.

We report the identification of a family of sequences located by in situ hybridisation to the centromeres of all the Triticeae chromosomes studied, including the supernumerary and midget chromosomes, the centromeres of all maize chromosomes and the heterochromatic regions of rice chromosomes. This family of sequences (CCS1), together with the cereal genome alignments, will allow the evolution of the cereal centromeres and their sites to be studied. The family of sequences also shows homology to the CENP-B box. The centromeres of the cereal species and the proteins that interact with them can now be characterised.

We report the identification of a family of sequences located by in situ hybridisation to the centromeres of all the Triticeae chromosomes studied, including the supernumerary and midget chromosomes, the centromeres of all maize chromosomes and the heterochromatic regions of rice chromosomes. This family of sequences (CCS1), together with the cereal genome alignments, will allow the evolution of the cereal centromeres and their sites to be studied. The family of sequences also shows homology to the CENP-B box. The centromeres of the cereal species and the proteins that interact with them can now be characterised.