Salt-resistant association of simian virus 40 T antigen with simian virus 40 DNA in nucleoprotein complexes

Characterization of SV40 T antigen associated with the nuclear matrix

Simian virus 40 (SV40) T antigen associated with the nuclear matrix of SV40-infected TC7 cells has been characterized. Pulse-chase studies on the turnover of T antigen in the different subcellular fractions show that T antigen turns over most rapidly in its association with the purified SV40 nucleoprotein complexes (NPCs) and undergoes a slower rate of turnover in its association with the nuclear matrix. In contrast, turnover of SV40 T antigen in its association with the other subcellular fractions is not detected during the same period of time. Tryptic peptide maps establish that NPC-associated T antigen and nuclear matrix-associated T antigen are chemically related, in that they have two additional methionine-containing peptides that are not found in the majority of T antigen molecules. The association of T antigen with the nuclear matrix is independent of SV40 DNA replication since T antigen is still present in the nuclear matrix after a 1-hr shift-up of tsA58-infected cells to the nonpermissive temperature. In addition, T antigen is associated with the nuclear matrices of both C6 and Cos7 transformed cells, indicating that the association of T antigen with the nuclear matrix is independent of its ability to initiate and support SV40 DNA replication.

Both trans-acting factors and chromatin structure are involved in the regulation of transcription from the early and late promoters in simian virus 40 chromosomes

We isolated simian virus 40 (SV40) chromosomes from lytically infected CV-1 cells at various times during the late phase and transcribed them in vitro with either whole-cell or nuclear extracts of HeLa cells. The late promoter was 3- to 10-fold more active than the early promoter. With bare SV40 DNA templates, the early promoter was up to 10-fold stronger than the late promoter. The relative strengths of the early and late promoters on SV40 chromosomes were essentially independent of template concentration or length of the replicative phase of the infection. When monoclonal antibodies or antisera against T antigen (T Ag) were added to SV40 chromosomes or when T Ag, both free and chromatin bound, was removed by immunoprecipitation with anti-T, the activity of the late promoter remained essentially unchanged. Washing with 0.4 M NaCl removed T Ag from more than 90% of the mature chromosomes associated with T Ag. Transcription from the late promoter still predominated in the salt-washed T Ag-depleted chromosomes, even though there was a marked increase in early promoter activity. The depression of the early promoter could be reversed by adding the T Ag-containing extract back to the depleted chromosomes. Extraction of SV40 chromosomes with 1.5 M NaCl resulted in a decrease in the activity of the late promoter and a further increase in the activity of the early promoter so that the relative amounts of early and late RNA synthesized were similar to those for bare SV40 DNA templates. Late RNA synthesis from bare SV40 DNA templates was stimulated by high-speed supernatants prepared from nuclear extracts of SV40-infected cells but not from those of uninfected cells. Pretreatment of the supernatants with anti-T did not alter the result. Our findings indicate that the activity of the early and late SV40 promoters is regulated by at least two different mechanisms at the chromosomal level. One is mediated by a subclass of T Ag bound to SV40 chromosomes which represses early SV40 transcription but has no effect on late transcription. A second level of regulation, involving a tightly bound trans-acting chromosomal factor and a stable nucleoprotein structure, favors the late promoter over the early promoter by up to 10-fold.