A human DNA telomeric fragment contains yeast ARS and mitotic stabilizing sequences

Molecular and cytological analysis of a 5.5 Mb minichromosome

Mammalian artificial chromosomes (MACs) provide a new tool for the improvement of our knowledge of chromosome structure and function. Moreover, they constitute an alternative and potentially powerful tool for gene delivery both in cultured cells and for the production of transgenic animals. In the present work we describe the molecular structure of MC1, a human minichromosome derived from chromosome 1. By means of restriction and hybridization analysis, satellite-PCR, in situ hybridization on highly extended chromatin fibres, and indirect immunofluorescence, we have established that: (i) MC1 has a size of 5.5 Mb; (ii) it consists of 1.1 Mb alphoid, 3.5 Mb Sat2 DNA, and telomeric and subtelomeric sequences at both ends; (iii) it contains an unusual region of interspersed Sat2 and alphoid DNAs at the junction of the alphoid and the Sat2 blocks; and (iv) the two alphoid blocks and the Sat2-alphoid region bind centromeric proteins suggesting that they participate in the formation of a functional kinetochore.

Use of a human minichromosome as a cloning and expression vector for mammalian cells

A natural human minichromosome (MC1) derived from human chromosome 1 was shown to be linear and to have a size of 5.5 Mb. Human IL-2 cDNA and the neo gene were co-transfected into a MC1-containing human-CHO hybrid cell line. Integration of the foreign genes was directed to the pericentromeric region of MC1 by co-transfection of chromosome 1-specific satellite 2 DNA. A number of G418-resistant transfectants were obtained and expression of IL-2 was determined. FISH analysis demonstrated co-localization in the minichromosome of the IL-2 gene and of the satellite 2 DNA. An IL-2-producing clone was used in cell fusion experiments with IL-2-dependent murine CTLL cells to generate CTLL-human hybrids containing the modified minichromosome (MC1- IL2 ). The hybrids were able to grow in medium lacking IL-2 for 17 mean population doublings (MPD), indicating that expression of the cytokine was sufficient to relieve the IL-2 dependence of CTLL proliferation. Endogenous IL-2 production delayed the onset of apoptosis in the IL-2-dependent CTLL cells. Mitotic stability was shown to be 100% in the human-CHO hybrids and 97% per MPD in CTLL cells. These results demonstrate that a natural human minichromosome can be utilized as a cloning and expression vector for mammalian cells and that the MC1 minichromosome can be engineered to deliver IL-2 to two types of cells, fibroblasts and lymphocytes.

In vivo generation of linear plasmids with addition of telomeric sequences by Histoplasma capsulatum

Histoplasma capsulatum is a dimorphic pathogenic fungus that is a major cause of respiratory and systemic mycosis. We previously developed a transformation system for Histoplasma and demonstrated chromosomal integration of transforming plasmid sequences. In this study, we describe another Histoplasma mechanism for maintaining transforming DNA i.e. the generation of modified, multicopy linear plasmids carrying DNA from the transforming Escherichia coli plasmid. Under selective conditions, these linear plasmids were stable and capable of retransforming Histoplasma without further modification. In vivo modification of the transforming DNA included duplication of plasmid sequence and telomeric addition at the termini of linear DNA. Apparently Histoplasma telomerase, like that of other organisms such as humans and Tetrahymena, is able to act on non-telomeric substrates. The terminus of a Histoplasma linear plasmid was cloned and shown to contain multiple repeats of GGGTTA, the telomeric repeat unit also found in vertebrates, trypanosomes, and slime moulds.