The CENTROMERE1 (CEN1) region of Arabidopsis thaliana: architecture and functional impact of chromatin

We have analysed the centromere 1 (CEN1) of Arabidopsis thaliana by integra tion of genetic, sequence and fluorescence in situ hybridisation (FISH) dat a. CEN1 is considered to include the centromeric core and the flanking left and, right pericentromeric regions, which are distinct parts by structural and/or functional properties. CEN1 pericentromeres are composed of differe nt dispersed repetitive elements, sometimes interrupted by functional genes . In contrast the CEN1 core is more uniformly structured harbouring only tw o different repeats. The presented analysis reveals aspects concerning distribution and effects of the uniformly shaped heterochromatin, which covers all CEN1 regions. A l ethal mutation tightly linked to CEN1 enabled us to measure recombination f requencies within the heterochromatin in detail. In the left pericentromere , the change from eu- to heterochromatin is accompanied by a gradual change in sequence composition but by an extreme change in recombination frequenc y (from normal to 53-fold decrease) which takes place within a small region spanning 15 kb. Generally, heterochromatin is known to suppress recombinat ion. However, the same analysis reveals that left and right pericentromere, though similar in sequence composition, differ markedly in suppression (53 -fold versus 10-fold). The centromeric core exhibits at least 200-fold if n ot complete suppression. We discuss whether differences in (fine) compositi on reflect quantitative and qualitative differences in binding sites for he terochromatin proteins and in turn render different functional properties. Based on the presented data we estimate the sizes of Arabidopsis centromere s. These are typical for regional centromeres of higher eukaryotes and rang e from 4.4 Mb (CEN1) to 3.55 Mb (CEN4).