The poly(adenylic acid).cntdot.protein complex is restricted to the nonpolysomal messenger ribonucleoprotein of Physarum polycephalum

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Abstract
The distribution of poly(adenylic acid) [poly(A)].cntdot.protein complexes in the polysomal and nonpolysomal messenger ribonucleoprotein (mRNP) fractions of P. polycephalum was examined in the present study. Poly(A)-containing components released from the nonpolysomal mRNP by RNase digestion were quantitatively adsorbed to nitrocellulose filters at low ionic strength, were highly resistant to micrococcal nuclease under conditions in which free poly(A) was completely degraded, and sedimented as a 10-15S particle which was disrupted by sodium dodecyl sulfate and protease treatment. These are characteristics of the poly(A).cntdot.protein complex. In contrast, poly(A)-containing molecules released from the polysomes by RNase were refractive to nitrocellulose, were completely sensitive to micrococcal nuclease, and sedimented at 2-4 S, identical with the sedimentation exhibited by protein-free poly(A). Examination of the poly(A) sequences present in polysomal and nonpolysomal mRNP by polyacrylamide gel electrophoresis showed that the former contained only very short sequences, averaging .apprx. 15 nucleotides, while the latter exhibited only much longer segments, averaging .apprx. 65 nucleotides. Poly(A).cntdot.protein complexes are restricted to the nonpolysomal mRNP of Physarum; the limiting factor in complex formation may be the length of the available poly(A) binding site.