We have purified distinct complexes of nine to 12 proteins [referred to as BRG1‐associated factors (BAFs)] from several mammalian cell lines using an antibody to the SWI2‐SNF2 homolog BRG1. Microsequencing revealed that the 47 kDa BAF is identical to INI1. Previously INI1 has been shown to interact with and activate human immunodeficiency virus integrase and to be homologous to the yeast SNF5 gene. A group of BAF47‐associated proteins were affinity purified with antibodies against INI1/BAF47 and were found to be identical to those co‐purified with BRG1, strongly indicating that this group of proteins associates tightly and is likely to be the mammalian equivalent of the yeast SWI‐SNF complex. Complexes containing BRG1 can disrupt nucleosomes and facilitate the binding of GAL4‐VP16 to a nucleosomal template similar to the yeast SWI‐SNF complex. Purification of the complex from several cell lines demonstrates that it is heterogeneous with respect to subunit composition. The two SWI‐SNF2 homologs, BRG1 and hbrm, were found in separate complexes. Certain cell lines completely lack BRG1 and hbrm, indicating that they are not essential for cell viability and that the mammalian SWI‐SNF complex may be tailored to the needs of a differentiated cell type.

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Upon entry into a host cell, retroviruses direct the reverse transcription of the viral RNA genome and the establishment of an integrated proviral DNA. The retroviral integrase protein (IN) is responsible for the insertion of the viral DNA into host chromosomal targets. The two-hybrid system was used to identify a human gene product that binds tightly to the human immunodeficiency virus-type 1 (HIV-1) integrase in vitro and stimulates its DNA-joining activity. The sequence of the gene suggests that the protein is a human homolog of yeast SNF5, a transcriptional activator required for high-level expression of many genes. The gene, termed INI1 (for integrase interactor 1), may encode a nuclear factor that promotes integration and targets incoming viral DNA to active genes.

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Chromatin organization plays a key role in the regulation of gene expression. The evolutionarily conserved SWI/SNF complex is one of several multiprotein complexes that activate transcription by remodelling chromatin in an ATP-dependent manner. SWI2/SNF2 is an ATPase whose homologues, BRG1 and hBRM, mediate cell-cycle arrest; the SNF5 homologue, INI1/hSNF5, appears to be a tumour suppressor. A search for INI1-interacting proteins using the two-hybrid system led to the isolation of c-MYC, a transactivator. The c-MYC-INI1 interaction was observed both in vitro and in vivo. The c-MYC basic helix-loop-helix (bHLH) and leucine zipper (Zip) domains and the INI1 repeat 1 (Rpt1) region were required for this interaction. c-MYC-mediated transactivation was inhibited by a deletion fragment of INI1 and the ATPase mutant of BRG1/hSNF2 in a dominant-negative manner contingent upon the presence of the c-MYC bHLH-Zip domain. Our results suggest that the SWI/SNF complex is necessary for c-MYC-mediated transactivation and that the c-MYC-INI1 interaction helps recruit the complex.

The hepatitis 6 virus can be found in the serum and liver of some hepatitis B virus patients. We now report that the RNA genome of serum-derived 6 virus is single-stranded and circular. Livers of infected chimpanzees or woodchucks contained as many as 300,000 copies of genomic strand RNA per average cell, and at least some of this RNA had a circular conformation. Also present in the livers were RNA species complementary to the virion RNA. The genomic RNA was 5-22 times more abundant than this antigenomic strand. Some of the antigenomic RNA was complexed with genomic RNA, as evidenced by the fact that at least 34% of the antigenomic RNA was resistant to digestion with either RNase A in 0.3 M NaCl or S1 nuclease. Some of the antigenomic RNA was in a circular conformation. These and other fmdings showed that the structure and replication of hepatitis 6 virus are in many ways similar to those of the previously described plant viroids, virusoids, and satellite RNAs.The human hepatitis 8 virus, as described by Rizzetto et al.(1), is considered to be a defective virus, in that its natural transmission has been detected only in the presence of hepatitis B virus. This dependence has been demonstrated by experimental transmission of the agent to chimpanzees chronically infected with hepatitis B (2, 3). It has also been extended by transmission studies to woodchucks in which the woodchuck hepatitis B virus provided the transmission function (4). Antibodies to the woodchuck hepatitis virus envelope have been used to show that the concomitant hepadnavirus infection provides the packaging for the 8 viral genome. The characteristic antigen of the 8 particle, the 8 antigen, is packaged, with the virion RNA, inside a 36-nmdiameter lipoprotein envelope in which is incorporated hepatitis B surface antigen (2, 5).Bonino et al. have previously reported that the RNA of the 8 particle is a 1.7-kilobase (kb) single-stranded species (5).Denniston et al. used this RNA to make a cDNA clone in pBR322, designated pKD3, with a 166-base 8-specific insert (6). As presented here, we have derived an independent cDNA library, screened it with pKD3, made strand-specific probes, and used these probes to study both the virion and intracellular RNA from livers of infected chimpanzees and woodchucks. Our findings on the structure and replication of the 8 genome support the previous conjecture (7) that the agent bears similarities to the viroids and virusoids of plant cells (8).MATERIALS AND METHODS Viral RNA. For most experiments, RNA was directly extracted from serum after dilution (1:20) into a buffer containing 10 mM Tris, 10 mM EDTA, 0.1% sodium dodecyl sulfate, and Pronase at 1 mg/ml. After digestion for 1 hr at 370C, the sample was extracted once with phenol and twice with ether and then collected by precipitation with ethanol, using dextran as carrier. In some cases the virions were first purified by centrifugation through a sucrose cushion (5) or by isopycnic centrifugation in cesium chloride (5). RNA was then extracted as described.Li...

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Yin-Yang-1 (YY1) regulates the transcription of many genes, including the oncogenes c-fos and c-myc. Depending on the context, YY1 acts as a transcriptional repressor, a transcriptional activator, or a transcriptional initiator. The yeast two-hybrid system was used to screen a human complementary DNA (cDNA) library for proteins that associate with YY1, and a c-myc cDNA was isolated. Affinity chromatography confirmed that YY1 associates with c-Myc but not with Max. In cotransfections, c-Myc inhibits both the repressor and the activator functions of YY1, which suggests that one way c-Myc acts is by modulating the activity of YY1.