Accumulation of MVM gene products is differentially regulated by transcription initiation, RNA processing and protein stability.

The accumulation and stability of minute virus of mice (MVMp) RNA and protein as well as comparative strengths of the two viral promoters have been analyzed in highly synchronous infections of murine A9 fibroblasts. Results indicate that there is a temporal phasing of the accumulation of MVM RNA and protein: the RNA products of the P4 promoter appear prior to the products of the P38 promoter and NS1 and NS2 appear prior to the capsid proteins. Total and cytoplasmic spliced RNA accumulate similarly, although there is a lag in cytoplasmic accumulation of about 2 hr. Total RNA contains abundant unspliced R1 and R3 which are confined to the nucleus. There is no detectable difference in the ratio of the various spliced versions of each RNA species throughout infection. R2 accumulates faster and in a greater amount than R1 in both total and cytoplasmic RNA even though they are generated from the same promoter. During this same period, however, NS1 and NS2 accumulate to similar levels during 1-hr pulses. The stabilities of all MVM RNA species produced at both 9 and 12 hr postrelease are equivalent. Late in infection R3 accumulates faster and in greater amounts than the combined products of the P4 promoter, by approximately two- to threefold. This increase can be accounted for by an increase in the frequency of initiation from the P38 promoter, relative to P4, as assayed by nuclear run-on experiments. Therefore, the steady-state levels of the individual viral proteins during infection is controlled by specific regulation at the level of the initiation of transcription, RNA processing, and protein stability.