Heterochromatin revealed

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Large stretches of the genomes of plants and animals are taken up with heterochromatin, which has distinctive molecular and cytological properties, when compared with the gene-rich euchromatin. Heterochromatin is singularly hard to clone and sequence, and is therefore largely ignored in large-scale genome-sequencing efforts. However, it is well known to have important biological properties, such as supporting centromere function and causing long-range position effects on gene activity. These papers on a segment of the Arabidopsis genome offer a welcome look inside the ‘black box’ of a heterochromatic knob on the 4S chromosome arm, which is the first piece of cytologically recognizable heterochromatin to be fully sequenced. The region is ∼500 kb across and, at its core, contains a tandem array of a 2-kb transposon-related repeat. This is surrounded by complex arrangements of retrotransposons and DNA transposons from several different families, some of which have not previously been described in Arabidopsis. Almost no predicted or expressed genes are found in the sequence, and none at all at its core. Examination of genetraps 1 The CSHL/WUGSC/PEB Arabidopsis Sequencing ConsortiumCell. 2000; 100: 377-386 Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar revealed that 33 had landed in flanking euchromatin, but none in the heterochromatin. This could be because of a silencing effect such that integrations in the heterochromatin are not detected, but might also be caused by a tendency for genetraps to integrate only around expressed genes. Recombination in the region is also strikingly correlated with the presence of expressed genes, being reduced tenfold or more in the heterochromatin.