Identification of nucleotide sequences and some proteins involved in polyadenylation of RNA transcribed by Pol III from SINEs.

We have previously reported that not only transcripts of RNA polymerase II (pol II), but also one type of RNA transcribed by RNA polymerase III (pol III), undergo AAUAAA-dependent polyadenylation. Such an unusual feature is inherent in Short Interspersed Elements (SINEs) from genomes of certain mammals. For polyadenylation of its transcript, SINE should contain, besides an AATAAA hexamer and a transcription terminator, two specific regions: β, located downstream of box B of a promoter, and τ, preceding AATAAA. Here, using nucleotide substitutions in SINEs B2 (mouse) and Ves (bat), we identified nucleotides of β regions necessary for polyadenylation of their transcripts. These sequences (β signals) are the following: ACCACATgg in B2 and GGGCATGT in Ves. Using this approach, we identified τ signal of SINE B2 (GCTACagTGTACTTACAT), where TGTA tetramer is most important for polyadenylation. In Ves, τ region is a long polypyrimidine motif which is able to interact with PTB protein in Ves transcripts. We demonstrated by knockdown that B2 and Ves transcript polyadenylation is performed by canonical poly(A) polymerase with the participation of proteins CSPF-160 and Fip1, the known factors of mRNA polyadenylation. We also showed that a factor CFIm partaking in polyadenylation of many mRNAs, is involved only in polyadenylation of B2 transcripts. CFIm seems to interact with τ signal of В2 RNA and thereby facilitates the recruiting of other proteins engaged in polyadenylation. Thus, SINEs utilize at least some proteins involved in polyadenylation of pol II transcripts to polyadenylate their pol III transcripts.