A second major class of Alu family repeated DNA sequences in a primate genome.

We have analyzed repetitive DNA sequences in a prosimian, Galago crassicaudatus, and found that there are two distinct, highly repetitive families of sequences related to the human Alu family. The Type I family is closely analogous to the human Alu Family. The Type II family of repeats, which appears to be present in higher copy number, has a right half that is almost identical to the Type I family. However, the left half of the Type II sequence shows only limited homology to the galago Type I or human Alu families. A comparison of homologous sequences in the left half indicated that they are centered in regions of the Alu family which function as RNA polymerase III promoters. We have also observed at least one example of a Type II left half that was integrated into the genome independent of the Alu family right half sequence. The Type II family appears to be of much more recent evolutionary origin than the Type I and may have arisen by the independent integration of a RNA polymerase III promoter adjacent to the right half of a Type I Alu family sequence.