The presence and synthesis of c‐myc protein and mRNA in the cell cycle has been studied. We find that c‐myc mRNA is present, at equivalent levels, at all times in the cell cycle with the possible exception of mitosis. Furthermore, we demonstrate that this mRNA is transcribed in both G1 and G2 phases. An analysis of the c‐myc protein in vivo shows that de novo synthesis occurs in G1 and G2 and the protein turns over with a half‐life of approximately 20‐30 min in both phases. Furthermore, the level of c‐myc protein rapidly increases in cell populations when they re‐initiate the cell cycle, thereafter decreasing as the culture reaches quiescence. The results therefore suggest that expression of c‐myc can be rapidly modulated and that it is activated during the G0 to G1 transition, but is expressed thereafter in the cell cycle.

A measles virus (MV) genome originally derived from brain cells of a subacute sclerosing panencephalitis patient expressed in IP-3-Ca cells an unstable MV matrix protein and was unable to produce virus particles. Transfection of this MV genome into other cell lines did not relieve these defects, showing that they are ultimately encoded by viral mutations. However, these defects were partially relieved in a weakly infectious virus which emerged from IP-3-Ca cells and which produced a matrix protein of intermediate stability. The sequences of several cDNAs related to the unstable and intermediately stable matrix proteins showed many differences in comparison with a stable matrix protein sequence and even appreciable heterogeneity among themselves. Nevertheless, partial restoration of matrix protein stability coul be ascribed to a single additional amino acid change. From an examination of additional genes, we estimated that, on average, each MV genome in IP-3-Ca cells differs from the others in 30 to 40 of its 16,000 bases. The role of extreme variability of RNA virus genomes in persistent viral infections is discussed in the context of the pathogenesis of subacute sclerosing panencephalitis and of other human diseases of suspected viral etiology.

Measles virus matrix protein expression is restricted in the persistently infected brain cells of patients with the chronic neurological disease subacute sclerosing panencephalitis (SSPE). Prior studies of the nature of this restriction have identified polyadenylylated matrix gene-encoded RNA transcripts unable to direct effective translation. The defective nature of these mRNAs readily accounted for the inability to detect matrix protein in these persistently infected cells and suggested that in SSPE the restriction of matrix protein expression is achieved by preventing its synthesis. Recently, however, we reported evidence that matrix protein is synthesized in at least one example of this persistent infection, the SSPE cell line IP-3-Ca. In this case, failure of matrix protein to accumulate normally accounted for its restricted expression [Sheppard, R. D., Raine, C. S., Bornstein, M. B. & Udem, S. A. (1985) Science 228, 1219-1221]. To clarify the nature of the restriction displayed by IP-3-Ca cells, the synthesis and fate of the matrix protein of this SSPE cell line were examined in detail. No evidence of constraints on the efficiency of matrix protein mRNA transcription or translation was found. Instead, the restricted expression proved to be the result of rapid posttranslational degradation of matrix protein. We suggest that matrix protein gene mutations incurred in the course of genome replication are likely to be responsible for the diversity of observed mechanisms restricting matrix protein expression. In that event, the nature and position of the nucleotide substitution(s) would be the determinants of the level at which restricted expression is achieved.

Groups of 3, 17, and 28-day-old Swiss mice were inoculated intracerebrally with JHM virus, the neurotropic strain of mouse hepatitis virus (MHV), and studied serially by virologic and morphologic techniques. Beginning 2--5 days post-inoculation, all groups of infected mice developed CNS lesions which were destructive in the 3-day-old group and demyelinative in the 17 and 28-day-old animals. Infectious virus could be isolated from the brain, spinal cord, and liver. Electron microscopy demonstrated the virus to be pantropic in the CNS with virions occurring within ependymal cells, astrocytes, neurons, oligodendrocytes, endothelial cells, and cell of haematogenous origin. Giant cell formation was a constant feature. In regions of demyelination, oligodendrocytes exhibited a propensity to proliferative aberrant membrane. Myelin degradation was accompanied by membrane vesiculation and by the stripping action of macrophages. The lesions were not due to CNS elements in the inoculum since in animals inoculated with normal CNS suspensions from appropriate age groups failed to show lesions. The morphogenesis of JHM virions was followed ultrastructurally as was the formation of syncytia in the different cell types. In addition to delineating virus morphogenesis and myelin pathology, the results underscore the pantropic nature of JHM virus in the CNS, the synstemic nature of the infection, and that oligodendrocytes were the principal targets.