Analysis by microinjection of the biological effects of site-directed mutagenesis in cloned avian leukosis viral DNAs.

Cloned avian leukosis viral DNAs were mutagenized in the long terminal repeat, in the leader sequence for env mRNA, and at the poly-env junction. The effect of these mutations in the viral DNA upon its ability to direct virus production or env mRNA synthesis was analyzed by microinjecting the mutant DNAs into chicken embryo fibroblasts and into chicken cells transformed by the env-deficient Bryan strain of Rous sarcoma virus, respectively. The results indicated that: (i) addition of up to 8 base pairs 19 nucleotides upstream of the Hogness box did not block transcription; (ii) deletion of 26 base pairs, including the tRNA primer binding site, allowed synthesis of all viral products and participation in recombination, but replication was blocked; (iii) deletion of fewer than 50 base pairs 250 bases downstream of the long terminal repeat depressed expression of all viral genes; and (iv) deletion of most of the gag and pol genes did not inhibit env mRNA synthesis, but virion packaging of the unspliced transcript was inefficient.