tsl is a neurovirulent spontaneous temperature-sensitive mutant of Moloney murine leukemia virus TB which causes hindlimb paralysis in mice. Previously, it had been shown that the temperature-sensitive defect resided in the env gene. At the restrictive temperature, the envelope precursor polyprotein, gPr80env, is inefficiently processed intracellularly into two cleavage products, gp7O and Prpl5E. This inefficient processing of gPr8Oe"v is correlated with neurovirulence. In this study, it was shown that a single amino acid substitution, Val-25-Ile in gPr80"v, is responsible for the temperature sensitivity, inefficient processing of gPr8(W'v at the restrictive temperature, and neurovirulence of tsl. At the restrictive temperature, a steady-state level of nonprocessed, endoglycosidase H-sensitive gPr8Oenv remained in the endoplasmic reticulum of cells infected by tsl, but no endoglycosidase H-resistant gPr8Oe"v and only trace amounts of gp70 were detected in the infected cells. Since the host cell-encoded processing protease resides in the cis cisternae of the Golgi apparatus, inefficient processing of gPr80e"V at the restrictive temperature is. most likely due to inefficient transport of gPr8 env from the endoplasmic reticulum to the cis cisternae of the Golgi apparatus rather than due to misfolded gPr80rnV being a poor substrate for the processing protease at the restrictive temperature.

, a temperature-sensitive mutant of Moloney murine leukemia virus TB, induces hind-limb paralysis in mice. The DNA of both the tsl and Moloney murine leukemia virus TB env genes has been sequenced, and the encoded amino acid sequences have been deduced from the DNA sequences. Four amino acids in the tsl envelope protein have been identified which may be responsible for the tsl phenotype, which includes temperature sensitivity, nonprocessing of Pr8Oenv, and neurovirulence.

Early transposon (ETn) elements are 5.7-kb retrotransposons found in the murine genome. We have sequenced large portions of two ETn elements that have apparently transposed within the DNA of a murine myeloma cell line, P3.26Bu4. One of the transposed ETn elements has 5' and 3' long terminal repeats (LTRs) that are exact duplicates of each other and has a 6-bp target site duplication. These results suggest that this element, which inserted into an immunoglobulin gamma 1 switch region, moved by a retrotransposition process. Our nucleotide sequences confirm that individual ETn elements are very similar to one another and lack open reading frames. However, the ETn sequences reported here and those previously described differ significantly near their 5' LTRs, including 200 bp of weak similarity and 240 bp of complete disparity. Southern hybridization analysis suggests that both subfamilies of ETn sequences are represented many times in the mouse genome. The possibility that the disparate sequences have a role in transposition by ETn elements is discussed.

No abstract

tsl, a spontaneous temperature-sensitive mutant of Moloney murine leukemia virus TB, causes hind-limb paralysis in mice. A Val-25-*Ile substitution in gPr8O'ev is responsible for temperature sensitivity, inefficient processing of gPr8Oe"V, and neurovirulence. In this study, the Ile-25 in gPr80e"v was replaced with Thr, Ala, Leu, Gly, and Gln by site-directed mutagenesis of the codon for Ile-25 to generate a new set of mutant viruses, i.e., ts1-T,-A,-L,-G, and-E, respectively. The phenotypic characteristics of these mutant viruses differed from those of tsl. For each mutant, the degree of temperature sensitivity was correlated with the degree of inefficient processing of gPr8Oe"v, and the following rank order was observed for both parameters: tsl-E > tsl-G > tsl-L > tsl-A > tsl > tsl-T. In FVB/N mice, mutant viruses of low and intermediate temperature sensitivity and inefficiency in processing of gPr8Oe"v were neurovirulent and consistently caused mutant-specific disease proffles: tsl-T caused severe whole-body tremor, tsl-A generally caused hind-limb paralysis, and tsl-L generally caused a delayed-onset paraparesis. By 150 days postinfection, FVB/N mice that were infected with tsl-G and-E, mutants of high temperature sensitivity and inefficiency in processing of gPr8oe"v, had lymphoid leukemia instead of a neurological disease. These results suggest that the dynamics of gPr8Oe"v processing are important in determining the neurovirulent phenotype in vivo.