Murine leukemia viruses (MLVs or MuLVs) involve a group of retroviruses belonging to the gammaretroviral genus of the Retroviridae family and are named for their ability to cause tumor in mouse body. MuLVs include both exogenous and endogenous viruses. Exogenous viruses transmit from one host to another and some MuLVs could infect other vertebrates. Endogenous MLVs are integrated into the host's germ line and are passed from one generation to the next. Scientists have classified murine leukemia viruses into four categories based on host specificity and determined by the genomic sequence of viral envelope region. These four categories include ecotropic MLVs, Non-ecotropic MLVs, polytropic MLVs and modified polytropic MLVs. Separately, the ecotropic MLVs are capable of infecting the same species of host cells (mouse cells), and the non-ecotropic MLVs may be xenotropic which means that this species could infect different species; while polytropic or modified polytropic MLVs could infect a range of hosts including mice. Different strains of mice may have different numbers of endogenous retroviruses, and new viruses may arise as the result of recombination of endogenous sequences.
Nowadays, the genomes of endogenous and exogenous MuLVs have been completely sequenced. The viral genome is a single stranded, linear, positive-sense RNA which has around 8000 nucleotides because the replications of genomes of MuLVs keep a relatively low fidelity. Thus, different lengths of viral sequences can be found in a single host organism. MLV reverse transcriptases are thought to have a slightly higher fidelity than the HIV-1 RT.
The main structure of MLVs
The replication of genomes of MLVs starts from 5' to 3' orientation and their genomes contain gag, pol, and env regions which code structural proteins or enzymes including the RNA-dependent DNA polymerase and coat proteins. The genomic molecule contains a 5' methylated cap structure and a 3' poly-adenosine tail. Figure 1 shows the structure of Murine leukemia viruses:
Fig. 1 the Schematic Model of Murine Leukemia Viruses
Replicating MLVs have a positive sense, single-stranded RNA (ssRNA) genome that replicates through a DNA intermediate via the process of reverse transcription. The process of replication of MLVs showed in figure 2:
Fig. 2 The Schematic Procedures of MLVs’ Replication