John E. Dahlberg

Physical and genetic studies of Newcastle disease virus: evidence for multiploid particles.

Abstract Newcastle disease virus (NDV) grown in 10-day-old embryonated eggs was analyzed on sucrose gradients. Approximately 50% of the hemagglutinating particles were noninfectious and sedimented at a much slower rate than the infectious particles. The infectious particles, which exhibited a 2.5-fold range in sedimentation constants, were themselves highly heterogeneous. Further studies showed that the noninfectious particles contained no genomes, the bulk of the infectious particles contained one genome, and the larger infective particles contained two or more genomes. Although the overall ultraviolet sensitivity of NDV is nearly single-hit, the larger infective particles gave multihit inactivation kinetics, confirming that they contain more than one genome. Genetic tests showed that between 11 and 15% of the infective particles in a mixed yield from chick embryo cells were heterozygotes. On the assumption that (1) only integral numbers of genomes occur in NDV particles, and (2) the number of genomes per particle is randomly distributed, the average number of genomes per particle was calculated to lie between 0.5 and 0.8. The size distribution of the particles, and ultraviolet inactivation data on both the whole population and selected size classes are consistent with values in this range.

The caprine arthritis-encephalitis virus is a distinct virus within the lentivirus group

The genetic relatedness among viral genomes of caprine arthritis-encephalitis virus, visna virus, and progressive pneumonia virus, was determined. Whereas the genomic RNAs of two strains of visna virus are indistinguishable as reflected both by their annealing kinetics as well as by the thermal stability of the hybrids, the caprine arthritis-encephalitis virus and visna virus have only 30% of their nucleic acid sequences in common. Furthermore, within the homologous regions of the two viral genomes, there is a significant level (approximately 10%) of mismatched base pairs. This limited homology that exists between caprine arthritis-encephalitis virus and visna virus was lower than the sequence homology observed between the genomes of visna virus and progressive pneumonia virus, or between the genomes of caprine arthritis-encephalitis virus and progressive pneumonia virus. All this indicates that caprine arthritis-encephalitis virus is an additional distinct member of the Lentivirus group of the Retroviridae family.

Nucleotide sequence analysis of the long terminal repeat of integrated caprine arthritis encephalitis virus

The nucleotide sequence of the long terminal repeat (LTR) of caprine arthritis encephalitis virus (CAEV), a prototype lentivirus was determined. 6-bp directly repeated host cell sequences flank the 376-bp proviral LTRs. By comparison with other retroviral sequences, the CAEV LTR likely contains U3, R and U5 regions 207, 86 and 83 base-pairs in length, respectively. Sequences conforming to consensus transcriptional promoter sites were identified in the U3 region upstream of a potential transcription initiation site. A consensus polyadenylation signal is present 20 bases upstream of the putative R-U5 border and a potential poly(A) addition site. Sequence comparisons of the CAEV LTR with those of other retroviruses uncovered significant similarities with that of visna virus. No other global homologies with other retrovirus LTRs could be detected. CAEV utilizes a primer binding site complementary to lysine tRNA as does visna, AIDS associated retroviruses, and mouse mammary tumor virus. The putative primer for positive-strand DNA synthesis identified in the CAEV sequence is identical to that of visna virus and very similar to those of AIDS retroviruses and MMTV. In addition, a stretch that includes the TATA box of the CAEV LTR resembles closely the corresponding region in the AIDS retrovirus. These and other findings further strengthen the classification of AIDS retrovirus as a lentivirus.