Differential transcription of the repetitive R-element in various mouse cell types

Unusual expression of LINE-1 transposable element in the MRL autoimmune lymphoproliferative syndrome-prone strain

LINE-1 are endogenous mobile genetic elements that have dispersed and accumulated in the genomes of eukaryotes via germline transposition, with up to 100,000 copies in mammalian genomes. LINE-1 elements transpose by reverse transcription of their own transcript. Transposition requires synthesis of a full-length, sense-strand transcripts and proteins encoded by open reading frame (ORF) 1 and ORF2. Although severely repressed in most normal tissues, LINE-1 occasionally leads to disease by insertional mutagenesis. In the present study, Northern blot and in situ hybridization analyses revealed a template-strand transcription of LINE-1 ORF2 (encoding reverse transcriptase, RT) in lymphoid organs and the liver from MRL-+/+ and Fas-deficient MRL/lpr strains and their normal ancestors. While these sense transcripts are restricted to the nucleus in hepatocytes, they are also found in the cytoplasm in splenocytes. In contrast to transcription, ORF2 translation was detected only in MRL strains, as shown by the cytoplasmic labelling of splenic cells obtained with a monoclonal antibody recognizing the LINE-1 RT. This antibody coprecipitated two proteins of 45 and 12 kDa from MRL/lpr lymphoid organ lysates that were removed by pretreatment with anti-beta2-microglobulin antiserum, suggesting a structural association between a LINE-1 product and a major histocompatibility complex class I or class I-like molecule.

Structural organization and mammary-specific expression of the butyrophilin gene

Butyrophilin is a glycoprotein of the immunoglobulin superfamily that is secreted in association with the milk-fat-globule membrane from mammary epithelial cells. As a first step towards determining the possible function(s) of this protein in lactation, the mouse butyrophilin gene (Btn) has been cloned from a 129-ES cell genomic library. Over 14 kb of DNA was sequenced, including the entire transcriptional unit of the gene, and 4.6 kb and 1.1 kb of the 5' and 3' flanking region, respectively. In addition, the overall structure of the bovine gene (BTN) was determined by amplification of genomic DNA by the polymerase chain reaction. Both Btn and BTN comprise seven exons and six introns. The signal sequence and two immunoglobulin-like folds of the exoplasmic domain and the membrane anchor are encoded by separate exons, and the cytoplasmic domain is encoded by two short exons and a large terminal exon that also includes 3' untranslated sequence. The butyrophilin gene appears to have evolved from a subset of genes in the immunoglobulin superfamily and genes encoding the B30.2 domain, which is conserved in a family of zinc-finger proteins. Murine butyrophilin mRNA was detected specifically in the mammary gland by RNase protection analysis. Expression increased during the last half of pregnancy and was maximal during lactation. The 5' flanking region of Btn was analyzed for putative regulatory elements and is different from the promoters of other mammary-specific genes. Btn should be useful for determining the mechanisms underlying mammary-specific gene expression and potentially for the production of heterologous proteins in the milk of transgenic animals.

Identification of transcriptional regulatory activity within the 5' A-type monomer sequence of the mouse LINE-1 retroposon

LINE-1 (L1) is a retroposon found in all mammals. In the mouse, approximately 10% of L1 elements are full-length and can be grouped into two classes, A or F, based upon the type of monomer sequence repeated at the 5' end. In order to test for promoter activity in the 5' end of the A-type mouse L1 element, we cloned several different A-monomers into a promoterless chloramphenicol acetyltransferase (CAT) vector. The A-monomer constructs varied in their ability to regulate transcription of the CAT gene, exhibiting CAT activity 16-37% of that detected with the Rous sarcoma virus promoter and enhancer. A series of A-monomer deletions were tested for their ability to regulate CAT expression and gel retardation experiments were performed to identify regions of the A-monomer that may be involved in L1 transcriptional regulation. A-monomer sequences are usually found repeated 2-5 times at the 5' end of a full-length mouse L1. In the absence of long terminal repeats or an internal promoter, the tandem array of A-monomers may provide a mechanism for A-type L1 elements to generate transcripts containing transcriptional regulatory sequences.

Localization of a transcription start point within the human Hinf element

The Hinf family is a repetitive nucleotide sequence of the human genome. Certain structural features of Hinf DNA resemble the eukaryotic RNA polymerase II promoters. Therefore, we studied the ability of the Hinf element to function as a transcriptional promoter in mammalian cells. We placed the Hinf element upstream from the thymidine kinase-encoding (tk) sequence in a plasmid construct, pAC401 and introduced it into Ltk- mouse cells. The Hinf-tk plasmid was able to transform Ltk- cells to Tk+ phenotype. In another plasmid construct, pAC Hinf-neo, the Hinf element was inserted upstream from the sequence encoding neomycin (Nm) resistance (neo), and this plasmid was able to confer Nm resistance to HeLa cells. The nature of transcription initiation of the tk gene in four of the Tk+ clones transformed by pAC401 was examined by S1 nuclease analysis, and the transcription start point (tsp) for the tk gene in these clones was mapped within the Hinf element. The same tsp in the Hinf element was found in HeLa cells. Our studies show that the Hinf element functions as a weak promoter.