Using a family of synthetic tetradecamer oligonucleotides as a primer for cDNA synthesis and a second family of tetradecamers as a hybridization probe, we have prepared and isolated a cDNA clone of mouse myelin basic protein (MBP). The clone, pNZ111, corresponds to the region of the mRNA that codes for an amino acid sequence present in all four major forms of MBP (2)(3)(4)(5). Large amounts of the membrane are produced relative to the size of the myelin-forming cell. The process involves the synthesis of myelin-specific proteins followed by their integration into the growing membrane. During early myelinogenesis, membranes are formed that are loosely whorled around the axon. As myelin maturation proceeds, compaction of these membranes occurs to form the tight multilamellar structure characteristic of myelin.The myelin basic protein (MBP) constitutes about 33% of the protein of the myelin sheath in the central nervous system and 1-10% in the peripheral (6, 7). In most species, the predominant form of the MBP has a molecular mass of 18.5 kilodaltons (kDa), and the primary sequence of this protein is well conserved in animals as distantly related as the chicken, human, and rat (8-10). Two major forms of MBP predominate in rats and mice: one of these corresponds to the single major protein found in other species (18.5 kDa) and the other has a molecular mass of 14 kDa. In rats, the 14-kDa MBP is identical in sequence to the 18.5-kDa MBP, except for an internal deletion of residues 118-158 [numbered according to Martenson (11)] near the carboxyl terminus of the protein (10, 12). Two quantitatively minor forms of MBP have been found in rats and mice-a 21.5-kDa protein that is presumably identical to the 18.5-kDa MBP, except for the inclusion of a 25-30 amino acid sequence in the NH2-terminal half of the molecule and a 17-kDa MBP that bears the same relationship to the 14-kDa MBP (13).Myelination occurs postnatally in mouse brain beginning 8-10 days postpartum and continuing actively for 7-10 wk, with the maximal rate of myelin deposition occurring at about 18 days (4). Maximal synthesis of the 18.5-kDa and 14-kDa MBPs occurs at 18 days in vivo and coincides closely with the peak of myelin synthesis (14). During myelin maturation in the mouse, the proportions of the four MBPs in the membrane change (15-17) and this may be due to alterations in the relative rates of synthesis of the four proteins with age. With maturation, the proportion of the two minor MBPs (i.e., the 21.5 kDa and 17 kDa in myelin falls relative to the 18.5-kDa proteins and the 14-kDa/18.5-kDa MBP ratio increases dramatically. This latter change has been correlated with the relative rate of synthesis of these two proteins in vivo (14). Recently, Carson et al. (18) have identified trace amounts of larger forms of MBP that appear during the early stages of myelination. Identification of these larger forms has depended on immunoreagents and the relationships of the amino acid sequences to the major MBPs have, as yet, not been determined.We have r...

NTRE 7 is an avian retrovirus recombinant of the endogenous nononcogenic Rous-associated virus-0 (RAV-0) and the oncogenic, exogenous, transformationdefective (td) Prague strain of Rous sarcoma virus B (td-PrRSV-B). Oligonucleotide mapping had shown that the recombinant virus is indistinguishable from its RAV-0 parent except for the 3'-end sequences, which were derived from tdPrRSV-B. However, the virus exhibits properties which are typical of an exogenous virus: it grows to high titers in tissue culture, and it is oncogenic in vivo. To accurately define the genetic region responsible for these properties, we determined the nucleotide sequences of the recombinant and its RAV-0 parent by using molecular clones of their DNA. These were compared with sequences already available for PrRSV-C, a virus closely related to the exogenous parent tdPrRSV-B. The results suggested that the crossover event which generated NTRE 7 took place in a region -501 to -401 nucleotides from the 3' end of the td-PrRSV parental genome and that sequences to the right of the recombination region were responsible for its growth properties and oncogenic potential. These sequences included a 148-base-pair exogenous-virus-specific region that was absent from the RAV-0 genome and the U3 region of the long terminal repeat. Since the exogenous-virus-specific sequences are expected to be missing from transformation-defective mutants of the Schmidt-Ruppin strain of RSV, which, like other exogenous viruses, grow to high titers in tissue culture and are oncogenic in vivo, we concluded that the growth properties and oncogenic potential of the exogenous viruses are determined by sequences in the U3 region of the long terminal repeat. However, we propose that the exogenous-virus-specific region may play a role in determining the oncogenic spectrum of a given oncogenic virus.

We studied the effects of agents that raise intracellular cyclic AMP on synthesis of myelin components by cultured neonatal rat sciatic nerve Schwann cells and by continuous PNS cell lines derived from the fusion of neonatal rat sciatic nerve Schwann cells with rat RN22 Schwannoma. Treatment with N6,2'-O-dibutyryl cyclic AMP (dibutyryl cyclic AMP) caused a fourfold increase in Schwann cell incorporation of 35SO4 into sulfogalactosylceramide (sulfatide), and elicited a 10- to 20-fold increase in such incorporation by the continuous PNS cell lines; a similar effect on PNS cell line sulfatide radiolabelling was obtained with forskolin. Cultured Schwann cells expressed barely detectable levels of myelin P0 glycoprotein (P0) mRNA and myelin basic protein (MBP) mRNA. Treatment of the Schwann cells with axolemmal fragments or with dibutyryl cyclic AMP did not elicit a detectable increase in the levels of these mRNAs. The PNS cell lines constitutively expressed much higher levels of P0 mRNA than did the Schwann cells, and synthesized immunochemically demonstrable P0 glycoprotein, but did not express MBP. Treatment of the PNS cell lines with dibutyryl cyclic AMP markedly reduced expression of P0 mRNA and also diminished immunoreactive P0 glycoprotein. These PNS cell lines should prove useful for further studies of the control of Schwann cell differentiation.

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