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Genes of the Polycomb group (PcG) of Drosophila encode proteins necessary for the maintenance of transcriptional repression of homeotic genes. PcG proteins are thought to act by binding as multiprotein complexes to DNA through Polycomb group response elements (PREs); however, specific DNA binding has not been demonstrated for any of the PcG proteins. We have identified a sequence-specific DNA binding protein that interacts with a PRE from the Drosophila engrailed gene. This protein (PHO) is a homolog of the ubiquitous mammalian transcription factor Yin Yang-1 and is encoded by pleiohomeotic, a known member of the PcG. We propose that PHO acts to anchor PcG protein complexes to DNA.

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Plasmids belonging to the three HI plasmid incompatibility subgroups were characterized by the use of restriction enzymes and Southern transfer hybridization. A diversity of restriction enzyme patterns was noted among the HI subgroups, and a small amount of DNA homology was observed by probing these digests with a nick-translated HI1 plasmid. Within a single subgroup (HI1 and HI2), similar restriction enzyme patterns were noted. Plasmids of all three HI subgroups and the HII group had a guanine plus cytosine content of 49 to 50 mol%. The IncHII plasmid pHH1508a also showed some homology with the HIt probe. The DNA homology observed is probably responsible for common phenotypic properties encoded by these plasmids.Plasmids have been conveniently classified by incompatibility, the inability of related plasmids to stably coexist within the same host cell. In general, plasmids within a given incompatibility group show a high degree of DNA homology. Plasmids belonging to the H incompatibility group, however, do not necessarily demonstrate this property as determined by DNA-DNA hybridization studies with total plasmid DNA (5, 11). Based on these studies, the H plasmid group has been divided into three subgroups designated Hi, H2, and H3 such that members within a given subgroup show a high degree of DNA homology with one another, but not with members of the other two subgroups. The H3 subgroup to date comprises only a single member, MIP233 (11).Despite the lack of homology between members of the H plasmid group, they do share several common features. They are all large (greater than 100 megadaltons [Mdal]), have a thermosensitive mode of transfer (15), and determine morphologically and serologically related pili (2). These plasmids have played an important role in mediating antibiotic resistance in medically important pathogens (17,18,20 In the present study we investigated the molecular relatedness of these H plasmid subgroups by the use of restriction enzyme analysis and Southern transfer-hybridization techniques. Representative members of each subgroup were isolated, and their buoyant density and corresponding guanine plus cytosine (G+C) contents were determined. They were then digested with restriction enzymes. Although a diversity of fragment patterns was generated, trends were noted in the frequency of cutting by particular restriction enzymes.

The mouse cell line MOPC 315 is an IgA (AlI)-producing myeloma. We have studied a derivative of MOPC 315 that secretes normal AII chains but no heavy chain. This derivative, MOPC 315-26, was found to contain a rearranged Al gene in addition to a rearranged AU gene. The rearranged AI gene was cloned into bacteriophage A DNA and its structure was studied. The AI gene was found to have arisen by an aberrant recombination event that resulted in a single base insertion at the site of V-J region joining. In addition, the gene contained numerous point mutations in the vicinity ofthejunction ofthe V and J regions. Two point mutations occurred in the donor splice sequence normally used for the removal of the intron between the J and C regions, suggesting that the RNA synthesized from the aberrantly rearranged AI gene would be unable to undergo proper RNA splicing.The immunoglobulin system is a paradigm for the study of the control ofgene expression in mammalian cells. It is known that only one member ofan allelic pair ofgenes controlling the production offunctional immunoglobulin light and heavy chains is active in a cell. This is the phenomenon ofallelic exclusion (1-3). Moreover, only one type of immunoglobulin light chain (K or A) is synthesized in a single cell (type exclusion). Finally, only one ofthe A chain genes (Al, All, or AIII) is functional in a single cell (isotype exclusion).The molecular mechanisms that control these patterns of gene expression are not yet clear. The synthesis of a functional immunoglobulin chain requires prior rearrangement of the DNA that juxtaposes the variable (V) and joining (J) gene segments (4-6). Thus, control of expression could operate at the level ofDNA rearrangement. Joho and Weissman (7) have found that in normal B lymphocytes producing K chains approximately half of the K genes remain unrearranged. On the other hand, it has also been found that a substantial proportion oflymphoid cell lines examined have, in addition to a productive gene rearrangement, a second abnormal gene rearrangement. For example, most A chain producers carry rearranged K genes (1,(8)(9)(10). Thus, although the DNA rearrangement that juxtaposes the V and J gene segments is essential for immunoglobulin gene expression, it is also clear that, in the same cells, additional rearrangements can take place that do not result in the expression of a functional polypeptide chain.To understand the possible role that multiple gene rearrangements might play in controlling immunoglobulin gene expression, we have studied a derivative ofthe mouse myeloma cell line MOPC 315 that secretes only All light chains and have found that, indeed, both the All and Al genes have undergone rearrangement. To study the nature of these recombination events, we cloned the rearranged AI gene from these cells and examined its structure. The results showed that the gene had undergone aberrant rearrangement at the site ofrecombination of the V and J gene segments. In addition, when the nucleotide sequence was compared with that of ...