Bovine papillomavirus type 1 E1 replication-defective mutants are altered in their transcriptional regulation

Abstract

Bovine papillomavirus type 1 (BPV-1) is capable of replicating as a stable, high-copy-number plasmid in transformed rodent cells. The BPV-1 El open reading frame (ORF) encodes multiple functions involved in viral DNA replication. Mutations which disrupt the translational integrity of the El ORF disable the viral genome from replicating as a stable plasmid and result in the integration of the viral genome into the host chromosome generally at a low copy number. Despite the low copy number of the integrated genomes, BPV-1 El mutants transform rodent cells to anchorage independence very efficiently, at levels equal to or greater than that of wild-type (wt) BPV-1. Studies were performed to provide insight into why these low-copy-number, replicationdefective mutants are capable of expressing an equal or greater transformation potential than wt BPV-1. Analysis of viral RNA revealed higher rates of transcription per viral genome in cells harboring El mutated BPV-1 DNA than in cells containing wt BPV-1 DNA. Furthermore, the levels of viral RNA mapping the P89 promoter were found to be 15to 35-fold higher In cells transformed by El mutated DNAs compared with wt BPV-1 transformants. This promoter controls expression of the BPV-1 E6 transforming gene and is regulated by the viral E2 gene products. The studies presented in this report determined that the El mutants were perturbed in their E2 transcriptional regulation, suggesting a possible explanation for the observed P89 induction. Mutations throughout the El ORF, in either of the two regions previously identified as encoding distinct replication functions, were altered in viral transcription.

Proteins encoded by the bovine papillomavirus E1 open reading frame: expression in heterologous systems and in virally transformed cells

“…A function ascribable with certainty to the 72-kDa El protein is the down-regulation of transcription from the viral promoter P89 (27) and, consequently, of the rate of transformation of C127 cells (46). This effect was confirmed by complementing El mutants, with respect to their hightransformation-high-transcription phenotype, with constructs in which the entire El ORF was placed under long terminal repeat control (46), thus utilizing constructs analogous to the viral vectors we used to overproduce the protein.…”

“…In addition, M and R mutants were reported to complement each other for the sum of El replicative functions, implying the existence of two separate genes within El ORF (36). On the other hand, somewhat contradictory evidence that El mutants present a unique phenotype, increasing both transformation frequency and viral transcription irrespective of the location of the mutational alteration in either the R or M domain (27,46), has been presented. Furthermore, M and R mutants failed to complement each other for this property (46).…”

Transient Viral DNA Replication and Repression of Viral Transcription Are Supported by the C-Terminal Domain of the Bovine Papillomavirus Type 1 E1 Protein

“…The E1 protein initiates DNA replication by binding to the origin (6,33,36), and the E2 protein is a transcriptional transactivator that cooperatively binds to the origin with E1 (22,25,36). E1 also represses viral transformation (8,20) and can regulate viral gene expression (9,18). The E2 protein can activate transcription from several viral promoters (24).…”

Prevalence of deletions of YY1‐binding sites in episomal HPV 16 DNA from cervical cancers

“…This will break up down-regulation of P97 by E2 and may result in overexpression of E6 and E7 proteins (Zur Hausen, 1991). Bovinepapillomavirus-type-1 El mutants showed increased transcription and transforming activity (Lambert and Howley, 1988;Schiller et al, 1989) and experimental knock-out of the El gene of HPV 16 enhanced viral-immortalization capacity (Romanczuk and Howley, 1992). These data suggest that integration of the HPV genome may contribute to derepression of viral oncogenes and thus play an important role in tumor progression.…”