No abstract

Posttranslational modifications and remodeling of nucleosomes are critical factors in the regulation of transcription. Higher-order folding of chromatin also is likely to contribute to the control of gene expression, but the absence of a detailed description of the structure of the chromatin fiber has impaired progress in this area. Mammalian somatic cells contain a set of H1 linker-histone subtypes, H1 (0) and H1a to H1e, that bind to nucleosome core particles and to the linker DNA between nucleosomes. To determine whether the H1 histone subtypes play differential roles in the regulation of gene expression, we combined mice lacking specific H1 histone subtypes with mice carrying transgenes subject to position effects. Because position effects result from the unique chromatin structure created by the juxtaposition of regulatory elements in the transgene and at the site of integration, transgenes can serve as exquisitely sensitive indicators of chromatin structure. We report that some, but not all, linker histones can attenuate or accentuate position effects. The results suggest that the linkerhistone subtypes play differential roles in the control of gene expression and that the sequential arrangement of the linker histones on the chromatin fiber might regulate higher-order chromatin structure and fine-tune expression levels.

Human papillomaviruses (HPVs) establish long-term infections in patients. The mechanism for extrachromosomal HPV DNA persistence in cycling cells is unknown. We show that HPV origincontaining plasmids partition as minichromosomes, attributable to an association of the viral origin recognition protein E2 with mitotic spindles. ␣-, ␤-, and ␥-tubulins were pulled down with a tagged E2. The N-terminal transacting and C-terminal protein dimerization͞DNA binding domains independently associated with the spindles. We suggest that this E2 property enables these viruses to establish persistence. Its implication for HPV oncogenesis is presented.F or any extrachromosomal DNA virus to establish a persistent infection in cycling host cells, the viral genome must replicate and partition into both daughter cells during division. The E2 origin (ori)-binding protein of bovine papillomavirus type 1 (BPV-1) associates with mitotic chromosomes (1-3), thus providing a mechanism for viral DNA segregation. Comparable mechanisms have been demonstrated for the Epstein-Barr virus through the Epstein-Barr virus-encoded nuclear antigen 1 protein and the Kaposi's Sarcoma virus (human herpesvirus 8) through the latency-associated nuclear antigen 1 protein (4, 5). In contrast, the mechanism by which human papillomavirus (HPV) DNA partitions during cell division has not been elucidated. In this report, we demonstrate that HPV ori-containing DNA segregates as minichromosomes by association with mitotic spindles and this association is mediated by the HPV origin recognition protein E2.HPVs are medically important pathogens that establish persistent infections in long-living basal keratinocytes. Infections typically cause benign hyperproliferation of squamous epithelia in the form of cutaneous warts, laryngeal papillomas, and anogenital condylomata. Over time, infections can become subclinical, but may reactivate during episodes of immune suppression. The HPV genome is a double-stranded, circular DNA of Ϸ7,900 bp and replicates extrachromosomally in the nucleus of infected keratinocytes. Low copy numbers of the mucosotrophic HPV DNA plasmids are maintained in the basal and parabasal cells that divide, whereas the productive phase takes place only in postmitotic, differentiated cell strata and progeny virus shed within the sloughing superficial cells (6). Thus, it is paramount that, in either latent or active infections, HPV DNA must partition into the two daughters of dividing cells for viral persistence. To support viral DNA amplification in postmitotic cells, the viral E6 and E7 proteins inactivate the host tumor suppressor proteins p53 and pRB (retinoblastoma protein), respectively, reestablishing an S-phase environment. Elevated transcription of these oncogenes is normally limited to the differentiated compartment. However, if inappropriately expressed in the basal cells, such as during repeated wounding and healing, the high-risk HPV oncoproteins can promote excessive cell cycling and host chromosome instability. Indeed, a small fraction of ...