The degradation of poly(adenylic acid) [poly(A)] and its relationship to translation were examined in the cytoplasmic RNA of P. polycephalum. Poly(A) sequences averaging about 80 and 65 nucleotides in length are present in Physarum nuclear and cytoplasmic RNA, respectively. These RNA classes also contain an oligo(A) sequence. Analysis of the electrophoretic mobility and total quantity of cytoplasmic RNase A and T1-insensitive polynucleotides in a pulse-chase regime reveal that poly(A) degradation involves both a gradual shortening process, in which about 15 nucleotides are removed from this sequence, and a turnover process in which 95% of the radioactive poly(A) disappears. The shortening process, unlike the turnover process, is not responsible for the complete eradication of poly(A) since tracts of about 50 nucleotides in length appear in the steady-state cytoplasmic RNA and accumulate when the turnover process is suppressed. In contrast to the behavior of poly(A), the oligo(A) sequences did not appear to significantly decay during the chase. A 37.degree. C heat shock, which suppresses protein synthesis and disrupts polysome structure, does not interfere with either poly(A) degradation process, whereas cycloheximide, which hinders mRNA cycling through the polysome, substantially slows the turnover process but does not affect the shortening process. At least 2 processes, which exhibit differential linkage to the translation cycle, are apparently involved in poly(A) degradation during the aging of cytoplasmic RNA in Physarum.
