Specificity of reticulocyte initiation factors for the translation of globin messenger RNA

Isolation of rabbit reticulocyte initiation factors by means of heparin bound to sepharose

Passage of cell-free extracts of rabbit reticulocytes through heparin-Sepharose affinity columns results in the loss of the ability of the effluent to initiate protein synthesis. This is shown by the loss of response to added rabbit globin mRNA or to inhibitors of initiation of protein synthesis, such as heparin and aurin tricarboxylic acid, and by recovery of initiation activity by addition of protein retained and subsequently eluted from the columns. The effluent retains, however, the ability to elongate protein chains. Only 0.8% of the applied cell extract protein binds to heparin-Sepharose columns. This bound protein, which can be recovered by increasing the salt concentration of the eluting buffer, has initiation factor activity equal to that of a crude initiation factor preparation obtained from rabbit reticulocyte ribosomes by extraction with 0.5 M KCl. The protein patterns on polyacrylamide gels of the initiation factors prepared by either method are very similar and indicate a protein mixture, which may represent a complex. These data confirm that heparin interacts specifically with initiation factos, and indicate that heparin-Sepharose chromatography will simplify procedures for the preparation of initiation factors.

The cell-free translation of Rauscher leukemia virus RNA into high molecular weight polypeptides

Rauscher leukemia virus (RLV) 65-S RNA, 35-S mengovirus RNA and reticulocyte A-rich RNA each stimulated cell-free protein synthesis in a JLS-V5 cell derived S-30 system. rRNA, however, was not stimulatory in this system. Of the stimulated protein products only those synthesized in response to added RLV RNA were immune-precipitable with anti-RLV rabbit serum. Furthermore, cell-free incubations with pactamycin at a concentration which specifically inhibits initiation and not elongation prevented the stimulation of amino acid incorporation in response to added RLV RNA. Analysis of the polypeptides synthesized by the cell-free system in response to reticulocyte A-rich RNA, showed them to be globin-like and, therefore, also mRNA specific. The RLV RNA-directed product included at least two classes of polypeptides (mol. wts of 140 000-185 000 and 50 000-75 000) both of which were larger than the group specific polypeptides of mature virions. None of the internal structural polypeptides of mature virions were synthesized in response to RLV RNA. The large molecular weight, viral-specific polypeptides are candidate precursor polyproteins which may represent the translational products of a polycistronic mRNA with a single initiation site.