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. sinen sis, C. ichangensis, Poncirus trifoliata). The monomers are mostly 181 bp i n length with a GC-content between 60% and 68% (significantly higher than t he average GC-content of the citrus group genomes). Similarity among the re peats indicates that they belong to a satellite family that underwent speci es-specific modifications, which are reflected in the phylogenetic relation ships. Curvature provoked by dA-stretches of the repeats analyzed by gel sh ifts revealed structural conservation, even though the nucleotide sequences vary among species, thereby probably supporting the heterochromatic struct ure of stDNA. We show that the species-specific modification of the satelli te consensus involves changes in the position and number of dA tracts. The molecule shapes of satellite oligomeres predicted by computer modelling ind icate a superhelical structure of the tandem repeats which is in a good agr eement with the satellite sequence dendrogram. The contribution of DNA bend ing elements to the evolution of plant satellite repeats is discussed.