Molecular properties of a gag- pol- env+ murine leukemia virus from cultured AKR lymphoma cells.

We have described the isolation of a replication-defective murine leukemia virus from a culture of AKR lymphoma cells [Rein et al., Nature (London) 282:753-754, 1979]. To facilitate the characterization of this murine leukemia virus, we transmitted it to mink cells and analyzed its genome by restriction mapping of the mink cellular DNA. This genome resembled the Akv genome quite closely, but it had an additional KpnI cleavage site at 1.3 kilobase pairs from the 5' end of the provirus and a small (approximately 50-base-pair) deletion between 1.8 and 3.0 kilobase pairs from the 5' end. When we tested these mink cells by immune precipitation or by competition radioimmunoassay, we found that they synthesized gPr82env, but contained no detectable gag or pol proteins. It seems likely that the KpnI cleavage site at 1.3 kilobase pairs reflects an abnormal sequence at or near the beginning of the gag gene, which prevents gag or pol translation by introducing a frameshift or termination codon into this region.     

Molecular cloning and characterization of a leukemia-inducing myeloproliferative sarcoma virus and two of its temperature-sensitive mutants.

The myeloproliferative sarcoma virus (MPSV) induces extensive hematopoietic changes, including spleen foci in adult mice, and transforms fibroblasts in vitro. NRK nonproducer cell lines of MPSV and ts temperature-sensitive mutants were analyzed by restriction enzyme digestion and Southern blotting. EcoRI fragments containing the proviral DNAs of MPSV and two temperature-sensitive mutants and rat cellular sequences homologous to c-mos were molecularly cloned. By comparing restriction enzyme cleavage sites, it was shown that the MPSV genome consists only of sequences related either to Moloney murine leukemia virus or to the c-mos mouse oncogenic sequences. Two regions of fragment heterogeneity were observed: (i) in the defective pol gene, where MPSV and the two cloned temperature-sensitive mutants were different from Moloney murine sarcoma virus and from each other, although MPSV wild-type retained more of the pol gene than any of the Moloney murine sarcoma virus isolates; (ii) in the area 3' to the mos gene, which was identical in MPSV and its temperature-sensitive mutants but different from other Moloney murine sarcoma virus variants. Transfection of cloned MPSV DNA in RAT4 cells and virus rescue on infection with Friend murine leukemia virus yielded MPSV which transformed fibroblasts in vitro and also induced spleen foci in adult mice, thus proving that both properties are coded by the same viral genome.     

Nucleotide sequence of AKV murine leukemia virus.

AKV is an endogenous, ecotropic murine leukemia virus that serves as one of the parents of the recombinant; oncogenic mink cell focus-forming viruses that arise in preleukemic AKR mice. I report the 8,374-nucleotide-long sequence of AKV, as determined from the infectious molecular clone AKR-623. The 5'-leader sequence of AKV extends to nucleotide 639, after which lies a long open reading frame encoding the gag and pol gene products. The reading frame is interrupted by a single amber codon separating the gag and pol genes. The pol gene overlaps the env gene within the 3' region of the AKV genome. The nucleotide sequence of the 5' region of AKV reveals the following features. (i) The 5'-leader sequence lacks any AUG codon to initiate translation of gPr80gag, suggesting that gPr80gag is not required for the replication of AKV. (ii) A short portion of the leader region diverges in sequence from the closely related Moloney murine leukemia virus and appears to be related to a sequence highly repeated in eucaryotic genomes. (iii) As in Moloney murine leukemia virus, there is a potential RNA secondary structure flanking the amber codon that separates the gag and pol genes. This structure might function as a regulatory protein binding site that controls the relative levels of synthesis of the gag and pol precursors. The nucleotide sequence of the 3' region of AKV is compared with sequences reported previously from both infectious and noninfectious molecular clones of AKV.     

Suppression of UAA and UGA termination codons in mutant murine leukemia viruses.

Genomes of mammalian type C retroviruses contain a UAG termination codon between the gag and pol coding regions. The pol region is expressed in the form of a gag-pol fusion protein following readthrough suppression of the UAG codon. We have used oligonucleotide-directed mutagenesis to change the UAG in Moloney murine leukemia virus to UAA or UGA. These alternate termination codons were also suppressed, both in infected cells and in reticulocyte lysates. Thus, the signal or context inducing suppression of UAG in wild-type Moloney murine leukemia virus is also effective with UAA and UGA. Further, mammalian cells and cell extracts contain tRNAs capable of translating UAA and UGA as amino acids. To our knowledge, this is the first example of natural suppression of UAA in higher eucaryotes.     

Two base changes restore infectivity to a noninfectious molecular clone of Moloney murine leukemia virus (pMLV-1).

The complete nucleotide sequence of a molecular clone of Moloney murine leukemia virus (pMLV-1) has previously been reported (Shinnick et al., Nature [London] 293:543-548, 1981). However, pMLV-1 does not generate infectious virus after transfection into cells (Berns et al., J. Virol. 36:254-263, 1980). The lesion in pMLV-1 has been localized by determining the biological activity of recombinants containing DNA from an infectious clone of Moloney murine leukemia virus (pMLV-48) and pMLV-1. Replacement of a 1.0-kilobase pair region which spans the gag-pol junction of pMLV-1 with the corresponding DNA fragment from the infectious clone restores its infectivity. Nucleotide sequence analysis of this fragment obtained from the infectious clone (pMLV-48) and pMLV-1 reveals two single base pair changes, one in the p30gag gene and the other in the 5' end of the pol gene. The mutation in the pol gene does not affect the production of infectious virus but renders them XC negative, whereas the mutation in the gag gene appears to be lethal. The complete nucleotide sequence of an infectious clone of Moloney murine leukemia virus can now be deduced.