The sequences homologous to major interspersed repeats B1 and B2 of mouse genome are present in mRNA and small cytoplasmic poty(A)⁺RNA

Abstract

Heavy nuclear RNA contains high amounts of transcripts from repetitive sequences B1 and B2. Cytoplasmic poly(A)+RNA and, particularly, polysomal poly(A)+RNA (mRNA) also include these sequences but in smaller amounts. The abundant 2 kb mRNA of mouse liver are found to have a B2 sequence. These sequences are also found in a new class of low-molecular-weight heterogeneous (200-400 nucleotide long) poly(A)+RNAs. These RNAs are located mostly in cytoplasm rather than in polysomes. The amount of small B2+ RNAs is noticeable larger than that of small B1+ RNAs. Tumour cells seem to contain more small B2 RNA than normal cells. The hybridization tests show that extended homology exists between the B1 sequence and 4.5S small nuclear RNA, which is predicted from comparison of their base sequences. Also, we have found homology between B1 and small cytoplasmic 7S RNA. Hybridization of B2 to sn or sc RNAs has not been observed, although the sequencing reveals partial homology between B2 and 4.5S sn RNAI /1/.

Most highly repeated dispersed DNA families in the mouse genome.

“…For Bi, we observed the 7S and 4.5S RNA transcripts described by others (30). Apart from the previously described 20S RNA transcript (31), no other discrete RNA transcripts hybridized to the B2 probe. The R and MIF-1 probes only hybridized to a very faint smear of transcripts.…”

“…We do not know whether these small B2 transcripts are synthesized by PollI or PolIII. The B2 sequence includes all of the necessary regulatory sites for initiation and termination by PollIl (31). Our transcription elongation analysis in nuclei has shown that both Bi and B2 sequences are included in Po/IlI as well as Polll transcripts in the liver ( Table 2).…”

“…The Polll transcripts are probably large RNAs, which include B1 and B2 in the untranslated regions or introns. B2 sequences have been reported in the 3' untranslated regions of several mRNAs (31,55). The small B2 transcripts seen in embryonal carcinoma cells and various tumor cells are poly(A)+ (Fig.…”

“…It has been proposed that most of the copies of Bi, B2, and R are pseudogenes which have arisen by reverse transcription of RNA intermediates (20,31,33). This idea was first introduced to explain the formation of pseudogenes for small nuclear RNAs and the Alu family of human repeated sequences (18,29,58).…”

Polymorphism in an androgen-regulated mouse gene is the result of the insertion of a B1 repetitive element into the transcription unit.

“…Both Bi and B2 repeats have the same approximate size, 130 and 160 base pairs (bp) respectively, are characterized by an A-rich tract at their 3' ends, are often flanked by short direct repeats at their site of insertion in the genome, and contain putative RNA polymerase III promoter regions (18,24). In vitro and in vivo transcription of a few individual Alu-type elements by RNA pol III has been reported (6,10,17,37).…”