and 7 factors, we have synthesized five peptides that are homologous to the corresponding sequence from the porcine or rat carboxyl-terminal region. Binding studies with the synthetic peptides suggest that amino acid residues 434-440 of ,Btubulin are crucial for the interaction of MAP2 and 7 factors.

The heterogeneity of 7 microtubule-associated proteins from rat brain is developmentally determined. Newborn rat brain contains two Tpolypeptides (To) with somewhat different molecular weights than the five 7 components associated with microtubules from 12-day-old brain (712). T and T12 are immunologically related and crossreact with antibodies against rI2 proteins. Enrichment of the 7 mRNA was achieved by prior hybridization of unfractionated poly(A)-containing mRNA to cDNA preparations containing tubulin and actin sequences. The remaining unhybridized mRNA was further fractionated by electrophoresis on methylmercury hydroxide agarose gels. Experiments involving cell-free translation of mRNA indicated that the major differences in the composition of x proteins from newborn and developing brain are controlled at the mRNA level. The mRNA from newborn rat brain directed the synthesis of five 7 proteins, two ofwhich are specific for newborn brain, whereas the other three forms are characteristic ofthe developing brain. Thus, the appearance in newborn brain of mRNA species specific for three T12 forms precedes the phase of the synthesis of these proteins in the cell. By contrast, mRNA from 12-day brain directed the synthesis of four X proteins specific for the developing brain, one of which is not synthesized by mRNA from newborn brain. None (11). The cell-free products were analyzed by NaDodSO4/10% polyacrylamide slab gel electrophoresis (12) or by two-dimensional gel electrophoresis using equal amounts of Ampholines (LKB) in the range pH 5-7 and pH 3.5-10 (13). Protease digestion of excised X bands from gels was performed according to Cleveland et al (14), using NaDodSO4/15-20% polyacrylamide gradient and 25 ng of Staphylococcus aureus V8 Abbreviation: MAP, microtubule-associated protein. 4892The publication costs ofthis article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.

No abstract

Rat liver tRNAphe contains a modified nucleoside which is similar in structure to the yeast Y-nucleoside. This peroxy Y-nucleoside is located immediately adjacent to the anticodon. Antibodies against yeast tRNAphe were shown to be specific to the Y-nucleoside. These antibodies bind to rat liver tRNAphe with a lower affinity than to yeast tRNAphe. Affinity chromatography on anti-Y antibody immobilized on Sepharose was used to determine the amount

Affinity chromatography on anti-Y (Y is a tricyclic imidazopurine to which is attached a complex four-carbon side chain) antibody immobilized to Sepharose was used to determine the proportion of rat liver tRNAPhe species containing the peroxy Y-nucleoside. Unfractionated Unfractionated mammalian tRNA was aminoacylated with labeled phenylalanine. The phenylalanyl-tRNA was then chemically acetylated to yield N-acetylphenylalanyl-tRNA. When this preparation was applied to the antibody column, between 6-10% of the radioactivity was not bound to the column, indicating a deficiency of peroxy Y-nuceloside in a minor isoaccepting tRNAPhe species. In contrast to normal tissues (including embryonic tissue), about 85% of the tRNAPhe from mouse neuroblastoma C-1300 or N-18 tumors lack the peroxy Y-base, a property which is not affected by tumor age. Rat liver labeled N-acetylphenylalanyl-tRNA preparations were resolved on Plaskon chromatography (RPC-5) into two minor peaks closely followed by a mojor component. A high proportion of the two minor tRNAPhe species was unable to bind to anti-Y antibodies. Upon mild acid treatment, the minor and major tRNAPhe species eluted simultaneously from Plaskon columns, at a much reduced salt concentration. These results would indicate that the two minor tRNAPhe species from rat liver as well as the major component contain a tricyclic imidazopurine base that differs from each other in its side chain. About 85% of the N-acetylphenylalanyl-tRNA from neuroblastoma was resolved by Plaskon chromatography as an early eluting peak. The position of this major neuroblastoma tRNAPhe species was not altered by mild acid treatment, and its elution position from the column almost coincides with that of acid-treated normal rat liver tRNAPhe. The latter results would suggest that most of the tRNAPhe chains from neuroblastoma lack the tricyclic imidazopurine of normal rat liver tRNAPhe, but are very close if not identical in primary nucleotide sequence.