Molecular and structural evolution of Citrus satellite DNA

Highly repeated satellite DNA (stDNA) of citric plants was characterized by cloning and sequencing 10–14 repeats of each plant (Citrus limon, C. sinensis, C. ichangensis, Poncirus trifoliata). The monomers are mostly 181 bp in length with a GC-content between 60% and 68% (significantly higher than the average GC-content of the citrus group genomes). Similarity among the repeats indicates that they belong to a satellite family that underwent species-specific modifications, which are reflected in the phylogenetic relationships. Curvature provoked by dA-stretches of the repeats analyzed by gel shifts revealed structural conservation, even though the nucleotide sequences vary among species, thereby probably supporting the heterochromatic structure of stDNA. We show that the species-specific modification of the satellite consensus involves changes in the position and number of dA tracts. The molecule shapes of satellite oligomeres predicted by computer modelling indicate a superhelical structure of the tandem repeats which is in a good agreement with the satellite sequence dendrogram. The contribution of DNA bending elements to the evolution of plant satellite repeats is discussed.