A cluster of repetitive elements within a 700 base pair region in the mouse genome

Approximately 39% of the clones from a BALB/c mouse genomic library hybridized with polyadenylated cytoplasmic RNA extracted from anemic mouse spleen. The DNA sequence of a portion of one such clone revealed the presence of three repetitive sequence elements within a 700 bp span. All three elements contain putative RNA polymerase III control regions oriented in the same direction and oligo(dA) tracts at their 3' ends. The first element is a member of the murine B1 family. A comparison of this element with other B1 family members indicates that the B1 family can be divided into two subclasses based on commonly held base changes and deletions. The second element within this 700 bp region may be a member of a new murine Alu family. Its structure is analogous to other murine Alu-equivalent sequences with respect to overall length, the presence of a 3' oligo(dA) tract and putative RNA polymerase III control regions. The third element is a murine type 2 Alu-equivalent sequence.     

Unusual sequences of two old, inactive human Alu repeats.

Two human Alu repeats terminating in an oligo(T) run rather than the usual A-rich 3' tail were isolated by library screening. Base sequence comparisons reveal that these unusual Alus are also exceptionally divergent from other Alu family members implying that they are evolutionarily old. Unlike other members of the family, they are not transcribed in vitro by RNA polymerase III (Pol III) suggesting a partial explanation for how Alu source genes might become inactive with age.