AN ANALYSIS OF RETROPOSITION IN PLANTS BASED ON A FAMILY OF SINES FROM BRASSICA-NAPUS

The identification of a family of SINE retroposons dispersed in the ge nome of oilseed rape Brassica napus has provided the basis for an evol utionary analysis of retroposition in plants. The repetitive elements (called S1(Bn)) are 170 bp long and occupy roughly 500 loci by haploid genome. They present characteristic features of SINE retroposons such as a 3' terminal A-rich region, two conserved polymerase III motifs ( box A and B), flanking direct repeats of variable sizes, and a primary and secondary sequence homology to several tRNA species. A consensus sequence was made from the alignment of 34 members of the family. The retroposon population was divided into five subfamilies based on sever al correlated sets of mutations from the consensus. These precise sepa rations in subfamilies based on ''diagnostic'' mutations and the rando m distribution of mutations observed inside each subfamily are consist ent with the master sequence model proposed for the dispersion of mamm alian retroposons. An independent analysis of each subfamily provides strong evidence for the coexpression of at least three subfamily maste r sequences (SMS). In contrast to mammalian retroposition, diagnostic positions are not shared between SMS. We therefore propose that SMS we re all derived from a general master sequence (GMS) and independently activated for retroposition after a variable period of random drift. P ossible models for plant retroposition are discussed.