Properties of dispersed, highly repeated DNA within and near the hamster CAD gene

Chromosome aberrations associated with CAD gene amplification in Chinese hamster cultured cells

Eleven sublines with increasing resistance to N-phosphonacetyl-L-aspartate (PALA) were isolated from the V79,B7 Chinese hamster cell line. Aspartate transcarbamylase activity and CAD gene copy number increased with increasing resistance of sublines. In situ hybridization with a DNA probe for the CAD gene showed that the amplified sequences resided in the terminal region of a marker chromosome with elongated q arms. This region stained homogeneously after G-banding. A high incidence of both numerical and structural chromosome aberrations was found in PALA-resistant cells. In hyperdiploid and polyploid cells, containing 2 copies of the marker chromosome, dicentrics were found at a very high frequency. As indicated by in situ hybridization and G-banding, they originated from a rearrangement involving 2 homologous marker chromosomes.

The Vi element. A transposon-like repeated DNA sequence interspersed in the vitellogenin locus of Xenopus laevis

A repeated DNA element in Xenopus laevis is described that is present in about 7500 copies dispersed throughout the genome. It was first identified in the 5' flanking region of one vitellogenin gene and was therefore named the Vi element. Seven copies are present within the vitellogenin gene region, three of them within introns of the genes A1, A2 and B2, and the other four copies in the gene flanking regions. Four of these copies have been sequenced. The Vi element is bounded by a well-conserved 13 base-pair inverted repeat; in addition, it is flanked by a three base-pair direct repeat that appears to be site-specific. The length of these four copies varies from 112 to 469 base-pairs; however, sequence homology between the different copies is very high. Their structural characteristics suggest that length heterogeneity may have arisen by either unequal recombinations, deletions or tandem duplications. Altogether, the characteristics and properties of the Vi element indicate that it might represent a mobile genetic element. One of the four copies sequenced is inserted close (position -535) to the transcription initiation site of the vitellogenin gene B2 in a region otherwise showing considerable homology with the closely related gene B1. Nevertheless, the presence of the Vi element does not seem to influence significantly the estrogen-controlled expression of gene B2. In addition, three alleles of this gene created by length polymorphism in intron 3 and in the Vi element inserted near the transcription initiation site are described.

Transcription of a long, interspersed, highly repeated DNA element in Xenopus laevis

We have analyzed the transcription of 1723, a long, repeated DNA element that is interspersed in the genome of Xenopus laevis (B. K. Kay and I. B. Dawid (1983) J. Mol. Biol. 170, 583-596). We have detected RNA homologous to 1723 in total cellular RNA from ovaries, embryos, liver, and cultured kidney cells. Transcripts from both strands of the element are present at similar concentrations in these different RNA preparations. In oocytes, approximately 100 pairs of lampbrush chromosome loops are active in the transcription of 1723 elements. The abundance of 1723 RNA increases during embryogenesis, with the highest level reached at the tadpole stage. From cellular fractionation studies, we conclude that 1723 transcripts are largely limited to the nucleus.

Effect of chromosomal position on amplification of transfected genes in animal cells

A single protein (CAD) contains the first three enzymatic activities of de novo uridine biosynthesis. The chromosomal location of CAD genes introduced into Chinese hamster ovary cells significantly affects the frequency and cytogenetic result of their amplification. The amplification frequency in one transformant is 100-fold that of the others; in another, amplification of donated genes inserted near a centromere results in chromosome instability and rearrangements.