A MASTER SEQUENCE RELATED TO A FREE LEFT ALU MONOMER (FLAM) AT THE ORIGIN OF THE B1 FAMILY IN RODENT GENOMES

The question of the origin of the B1 family of rodents is addressed. T he modern B1 elements are similar to the left Alu monomer, but with a 9 bp deletion and a 29 bp duplication. Search of databases for B1 elem ents that do not exhibit those modern features revealed sequence fragm ents that are very similar to the free left Alu monomers (FLAMs) descr ibed in the primate genomes. In addition, the analysis reveals element s that have 10 bp or 7 bp deletion in place of the 9 bp deletion but w ithout the 29 bp tandem duplication. The elements described define fam ilies of proto B1 elements (referred as PB1, PB1D10 and PB1D7) that ap peared before the first modern B1 element. A phylogenetic reconstructi on suggest that the origin of Alu and B1 families took place before th e divergence between the primate and the rodent lineages and that each family has followed different evolutionary routes since this radiatio n.