Propagation efficiency of MMTV transfer vector RNAs containing chimeric 5′ LTRs.

*JA10, MMTV gag/pol packaging expression vector; MD.G, vesicular stomatitis virus (VSV-G) envelope expression vector; pGL3, luciferase expression vector.**Propagation of the transfer vector RNA expressed as hygromycin resistance colony forming units (CFU)/ml of viral supernatant that was used to infect target cells. The data represents the mean of at least three independent transfection and infection experiments testing all mutants and was derived after normalization to the transfection efficiencies observed by luciferase expression from a co-transfected luciferase expression vector. SD, standard deviation.***Entire UTR refers to 160 bp excluding 17 bp of primer binding site (PBS).****The differences observed in the RNA propagation abilities of DA24 and NS12, both containing same amounts of 5′ UTR and gag sequences could be attributed to an artificially introduced SpeI site in NS12 at the junction of 5′ UTR and gag during cloning. This SpeI site may have destabilized some sequence/structural motifs important for MMTV transfer vector RNA packaging and propagation (compare DA24 and NS12 in Figure 5C).