5-Methylcytosine in the highly repetitive DNA's of the guinea pig (Cavia porcellus)

Organization of repetitive DNAs in the guinea pig genome

Investigation of restriction enzyme families observed in Bam HI and Eco RI digest of guinea pig DNA has led to the demonstration of a 4.0 kb cryptic satellite called MRICp which has 6300 copies and comprises 1% of the total genome. MRICp has been shown to encompass the Eco RI 4.0 kb family, the Bam HI 3.4 kb family and the Bam HI 3.7 kb family. The 1.5 kb Eco RI family is part of an L1 LINE that is present in the guinea pig and in other rodentia. It is part of a larger LINE, probably 6.5 to 7 kb in size, and some copies are embedded in satellite I DNA sequences.

Asymmetric distribution of exogenous thymidine among pyrimidine isostichs suggests compartmentalization of replicative DNA synthesis during regeneration in rat liver

The distribution of radioactivity among pyrimidine isostichs (or isoplyths) of DNA from 24-h regenerating rat liver was studied with [3H]Thd, [14C]orotate or with inorganic 32Pi. Expression of incorporated radioactivity as log10% of total radioactivity recovered for each of the 11 pyrimidine isostichs detected showed that radioactivity from [3H]Thd was asymmetrically distributed among the isostichs, i.e., 3H radioactivity failed to access regions of DNA yielding lower molecular weight pyrimidine isostichs as efficiently as it accessed regions yielding higher molecular weight pyrimidine isostichs. The thymine (T) content of isostichs exceeded that of cytosine (C), i.e., T/C ratios for the first 10 isostichs averaged 1.43 +/- 0.08 and 1.28 +/- 0.05, depending on the method of analysis; furthermore, the T/C ratio for isostich 1 was significantly higher than ratios for isostichs 2 through 10. Asymmetric distributions of [3H]Thd radioactivity also were seen at 18 or 30 h post-partial hepatectomy. Thus, radioactivity from [3H]Thd, a DNA precursor from the salvage pathway, failed to efficiently access lower molecular weight isostichs despite thymine enrichment, suggesting that thymine moieties were supplied from additional sources. Radioactivity from [14C]orotate accessed lower molecular weight pyrimidine tracts more efficiently than [3H]Thd, but less efficiently than it accessed higher molecular weight isostichs, resulting in an asymmetric distribution of 14C radioactivity. This result suggested that appreciable quantities of thymine and cytosine moieties utilized for DNA synthesis were supplied de novo, but other sources also were utilized. Radioactivity from 32Pi, a de novo precursor, was distributed symmetrically, i.e., the slope among lower molecular weight isostichs increased enough that it was indistinguishable from slopes for intermediate and higher molecular weight isostichs. Since 32P radioactivity among lower molecular weight isostichs reflects appreciable contributions of de novo phosphate moieties from both pyrimidine- and purine-containing deoxynucleoside triphosphates, opportunities for observing contributions of 32P radioactivity from pathways other than the de novo pathways appeared to lie beyond limits of detectability. The distribution of radioactivity from labeled DNA precursors among lower molecular weight pyrimidine tracts (a) indicate that thymine moieties are contributed by both salvage and de novo pathways; (b) support the possibility that cytosine moieties also are contributed by both pathways; and (c) support the 'replitase' concept for channeling dNTPs to replicating forks.

Nucleotide sequence definition of a major human repeated DNA, the Hind III 1.9 kb family

A human Hind III 1.9 kb repeated DNA fragment was isolated and cloned in pBR322. A cloned member that hybridized predominantly to the 1.9 kb Hind III band in a digest of whole human DNA was chosen for sequencing. It is an 1894 bp fragment that shows no significant internal repeats. Few pCG residues are observed in the sequence and there are numerous stop codons. Detailed sequence comparisons confirm this is a novel class of repeats that is not related to previously characterized human satellite DNAs or Alu sequences. At least a portion of the sequence described is conserved in evolution.