Biosynthesis and cytoplasmic distribution of small poly(A)-containing B2 RNA.

Previously, we described a small polyadenylated RNA predominantly located in cytoplasm and hybridizing with the ubiquitous B2 sequence of the mouse genome (Kramerov, D.A., Lekakh, I.V., Samarina, O.P. and Ryskov, A.P. (1982) Nucleic Acids Res. 10, 7477-7491). This 180-300 nucleotide long RNA was designated B2 RNA. Here, we demonstrate that B2 RNA is complementary to just one of the strands of cloned B2 sequence. The synthesis of B2 is rather resistant to ultraviolet irradiation of Ehrlich ascites carcinoma cells. The treatment of the cells with alpha-amanitin at a concentration completely blocking the formation of small nuclear RNAs U1, U2 and U3 does not interfere with the B2 RNA synthesis. These results suggest that B2 RNA formation is directly transcribed with the aid of RNA polymerase III, rather than being formed in the course of the processing of large RNA molecules which are known to contain a lot of B2 sequences. We also surprisingly found that the synthesis of up to 50% of long poly(A) +RNA in Ehrlich carcinoma cells is rather resistant to alpha-amanitin. The possible role of genetic elements including B2 sequences able to promote large RNA-polymerase III transcripts is discussed. B2 RNA in the cytoplasm is incorporated into the ribonucleoprotein particles, both small (12-18 S) and heavy. The latter probably correspond to informosomes. After deproteinization of heavy particles, a major part of B2 RNA still cosediments with mRNA and is split from it only after denaturation. We suggest that the B2 RNA of heavy ribonucleoproteins is associated with mRNA by short complementary stretches. About half of the B2 RNA is recovered in the cytoskeletal fraction. The possible role of B2 RNA in mRNA transport or in translation regulation is discussed.