The pattern of RNA synthesis was investigated in vitro in the sciatic nerve of the chicken. Nerve tissue from 14-day embryos, 17-day embryos, and 3-day chicks was incubated with [5-3H]-uridine for 0.5, 1.5, and 3.5 h. The RNA was isolated from the tissues and the purified preparations were fractionated by zone velocity centrifugation on sucrose gradients.The sedimentation properties of the rapidly labelled RNA changed in the three developmental stages. The proportion of the radioactivity in the light fractions of the RNA (5.8–17.2 S) increased with the age of the animals. In contrast, the specific activities of the heavy species decreased during nerve development. Pulse-chase experiments with unlabelled uridine and actinomycin D demonstrated the metabolic lability of the rapidly labelled RNA. The sedimentation properties of the pulse-labelled RNA of the cerebellum were the same in the three developmental stages.

No abstract

The pattern of the methylation of RNA was investigated in organ cultures of the sciatic nerve of the chicken. Nerve tissue from 14‐day embryos, 17‐day embryos and 3‐day‐ old chicks was incubated with [methyl‐3H]methionine or with [2‐14C]uridine and [methyl‐3H]methionine simultaneously for various periods of time. Subsequently, RNA was extracted from the tissues and the purified preparations were fractionated by polyacrylamide gel electrophoresis. The electrophoretic patterns of the rapidly labelled RNA changed during the three developmental stages. The incorporation of both uridine and the methyl groups from methionine was highest in the‘heavy’RNA species of the 14‐day embryonic nerve during the 0.5 and 1.0 h incubation periods. In contrast, in the nerves of 3‐day‐old chicks during a 0.5 h pulse with both precursors, methylation was almost entirely limited to the transfer RNA species. Furthermore, the incorporation of uridine in the nerves from 3‐day‐old animals revealed the presence of a heterogeneous population of rapidlylabelled, unmethylated species of RNA, most of which migrated between the smaller ribosomal RNA and transfer RNA components of the bulk RNA. The pattern of uridine incorporation and the methylation of the rapidly‐labelled RNA of the 17‐day embryonic nerve represented a transitional state between that of the 14‐day embryos and that of the 3‐day‐old chicks. The 17‐day embryonic stage of development corresponded to the phase of the onset of rapid deposition of myelin lipids in the sciatic nerve. Pulse‐chase experiments on the embryonic nerves indicated that a number of methylated precursors of ribosomal RNA and labile, heterogeneous, probably DNA‐like RNA were synthesized.

No abstract

To investigate certain biochemical aspects of myelination, a study was undertaken of the messenger‐like RNA in the nervous system of pre‐ myelinating 14‐day embryos and of myelinating 17‐day embryos and 3‐day chicks. The central and peripheral nervous systems of the chick were found to contain and to actively synthesize poly(A)+ RNA. RNA species binding to oligo(dT)‐cellulose contained a relatively high proportion of adenylate residues and were resistant to the actions of pancreatic and T1 ribonucleases. Preparations labeled by incubation with adenosine in vitro showed a decrease in the proportion of poly(A)+ RNA as the age of the animal increased, while preparations labeled in vivo exhibited the opposite trend. Polyacrylamide gel electrophoretograms of both in vivo and in vitro labeled pqeparations showed that the poly(A)+ fractions contained mainly heterodisperse RNA species. The average molecular size of poly(A)+ RNAs of purified polysomal fractions of nerve RNA from 3‐day chicks was smaller than 18S, whereas that of total poly(A)− RNA was larger than 18s. The proportion of poly(A)+ molecules larger than 18s was lower in the rapidly myelinating nerve tissues of 17‐day embryos and post‐hatching chicks than in those of premyelinating 14‐day embryos. Similar results were obtained for crude nuclear RNA fractions or RNA preparations fractionated under denaturing conditions. These results are consistent with previous work showing that the embryonic peripheral nerve contains a larger proportion of high‐molecular‐weight, messenger‐like RNA molecules than does nerve tissue from young chicks or adults.