REGULATION OF EXPRESSION OF MULTIPLY-INTEGRATED MOLONEY-MSV PROVIRAL SEQUENCES

Abstract

A series of morphological revertants of a Moloney murine sarcoma virus-transformed NRK nonproducer line, 6 O('+)H('-), was analyzed. The 6 O('+)H('-) genome contains three complete stably integrated proviruses. Thus reversion might result from lesions in single cellular components rather than multiple proviruses. Microdensitometry reveals that hybridization results suggesting a 5' LTR deletion in one of the three 6 O('+)H('-) proviruses are instead due to comigrating restriction fragments. Such a phenomenon is best accounted for by proposing that one provirus was duplicated along with a stretch of flanking rat DNA. All revertants have lost an entire provirus with retention of the remaining two. Virus expression is similar in 6 O('+)H('-) and revertants where individual transcripts are visible in a heterogeneous background. Revertants contain a viral transcript not detected in 6 O('+)H('-), this 35S species apparantly containing genetic information of the full-length 30S viral genome plus other information. Revertant proviruses are no more methylated and presumably no less active than the 6 O('+)H('-) proviruses. Revertant susceptibility to retransformation by Mo-MSV superinfection rules out possible cellular defects in RNA processing or transforming gene target sites. Mo-MuLV superinfection rescues from revertants low titers of transformation-competent virus. A spontaneously arising retransformant contains an additional provirus maintained as an extrachromosomal episome. It is proposed that the 6 O('+)H('-) genome contains two expressed competent MSV proviruses and one which is competent and silent. Its silence is maintained in revertants, reversed by a rare DNA rearrangement causing retransformation. Reversion results from the loss of one competent provirus and the generation of a point mutation in the other rendering it defective. Mo-MuLV rescues defective virus which occasionally back-mutates to generate competent virus. The lesions characterizing revertants affect MSV proviruses and not host cell components. Analysis of transformed and revertant genomes suggests a role of flanking host sequences in regulating the expression of some of the multiple proviruses.