| Popis: |
A method utilizing chromatography on poly (U) sepharose has been developed for isolating HnRNA molecules terminated in poly(A). The location within these molecules of oligo(U) fragments and of transcripts of repetitive DNA sequences has been determined by studies of the shorter 3′ poly(A) terminated sequences of HnRNA produced by cleavage with alkali. Poly(A) terminated HnRNA molecules longer than 20,000 nucleotides posses 2–3 units of 30 U nucleotides. In the shorter 3′ sequences released by alkaline breakage, there is a decrease of more than 90% in oligo(U) content, even in fragments over 12,000 nucleotides long. More than 90% of the oligo(U) fragments must lie more than 12,000 nucleotides from the 3′ poly(A) terminated end of HnRNA; they therefore cannot be located within or close to the region corresponding to mRNA, which is on average about 3000 nucleotides long. Denaturation and alkaline cleavage of HnRNA increases its rate of hybridization with DNA, implying that the molecule contains double stranded regions maintained by base pairing. Poly(A) terminated fragments of about 3000 or less nucleotides hybridize at about the same rate as messenger RNA. Fragments of up to about 8000 nucleotides in length hybridize some 3—4 times faster. No increase in hybridizing ability is obtained beyond this length. This suggests that some sequences in HnRNA close to the messenger part of the molecule are derived from repeated sequences of DNA; in the remaining 5′ part of the molecule, rapidly and less rapidly hybridizing sequences must alternate. |
