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[Universal orientation and 3'-terminal localization of repeated sequences in the B2 family of mRNA].

A cDNA library in pBR322 was prepared with cytoplasmic poly(A) + RNA from mouse liver cells. From 1 to 1.5% of clones hybridized to either B1 or B2 ubiquitous repetitive sequences. Several clones hybridizing to a B2 repeat were partially sequenced. The full-length B2 sequence was found at the 3'-end of abundant 20S poly(A) + RNA (designated as B2 + mRNA chi) within the non-coding part of it. B2 + mRNA chi is tissue-specific, concentrated in mouse liver polysomes and absent in cytoplasm of Ehrlich carcinoma cells. The B2 sequence seems to be located at the 3'-end of some other mRNAs as well. To determine the orientation of the B2 sequence in different RNAs, its two strands were labeled, electrophoretically separated, and used for hybridization with Northern blots containing nuclear, cytoplasmic and polysomal RNAs. In nuclear RNA, the B2 sequence is present in both orientations; in polysomal and cytoplasmic poly(A) + RNAs, only one ("canonical") strand of it can be detected. Low molecular weight poly(A) + B2 + RNA also contains the same strand of the B2 element. The conclusion has been drawn that only one of its strand can survive the processing. This strand contains promoter-like sequences and AATAAA blocks. The latter can be used in some cases by the cell as mRNA polyadenylation signals.