Occurrence and Sequence Complexity of Polyadenylated RNA in Squid Axoplasm

Axoplasmic RNA from the giant axon of the squid (Loligo pealii) comprises polyadenylated [poly (A) + ] RNA, as judged, in part, by hybridization to [ 3 H]polyuridine and by in situ hybridization analyses using the same probe. The polyadenylate content of axoplasm (0.24 ng/μg of total RNA) suggests that the poly(A) + RNA population makes up ∼0.4% of total axoplasmic RNA. Axoplasmic poly(A) + RNA can serve as a template for the synthesis of cDNA using a reverse transcriptase and oligo(deoxythymidine) as primer. The size of the cDNA synthesized is heterogeneous, with most fragments < 450 nucleotides. The hybridization of axoplasmic cDNA to its template RNA reveals two major kinetic classes: a rapidly hybridizing component (abundant sequences) and a slower‐reacting component (moderately abundant and rare sequences). The latter component accounts for ∼56% of the total cDNA mass. The rapidly and slowly hybridizing kinetic components have a sequence complexity of ∼2.7 kilobases and 3.1 × 10 2 kilobases, respectively. The diversity of the abundant and rare RNA classes is sufficient to code for one to two and 205, respectively, different poly(A) + RNAs averaging 1,500 nucleotides in length. Overall, the sequence complexity of axoplasmic poly(A) + RNA represents ∼0.4% that of poly(A) + mRNA of the optic lobe, a complex neural tissue used as a standard. Taken together, these findings indicate that the squid giant axon contains a heterogeneous population of poly(A) + RNAs.