Biomedical subjects

A A Amin

Publications and source records attributed to A A Amin.

21 records · Page 2Linked to original sources

PubMed1985

Autonomously replicating sequences from the non transcribed spacers of Tetrahymena thermophila ribosomal DNA.

The non-transcribed spacer (NTS) regions of the linear extrachromosomal palindromic rDNA from the ciliated protozoan Tetrahymena thermophila contain, in addition to sequences regulating transcription, the origin(s) of bidirectional replication as well as telomere associated sequences. These NTS regions function as autonomous replication sequences (ARS) in the yeast Saccharomyces cerevisiae (Kiss, G.B., Amin, A.A. and Pearlman, R.E., Mol. Cell Biol. 1: 535-543, 1981). We have now identified in the 5' NTS two adjacent but non-overlapping restriction fragments which function as ARS in yeast. These fragments contain 200 bp of duplicated sequence and include the in vivo origin of rDNA replication. The ARS in the 3' NTS has been subdivided into a sequence which allows high frequency transformation of yeast but with transformants extremely unstable when grown either under selective or non-selective conditions, and a sequence which appears to be required for plasmid maintenance yet not to be essential for high frequency transformation. Detailed analysis of the DNA sequences in these regions gives rise to interesting information about structural and functional features of the molecule.

Animals↗

PubMed1985

Conserved arrangements of repeated DNA sequences in nontranscribed spacers of ciliate ribosomal RNA genes: evidence for molecular coevolution.

We have analyzed the nucleotide sequences of the nontranscribed spacer (NTS) and transcription initiation and termination regions of the extrachromosomal rDNAs of the ciliated protozoans Tetrahymena thermophila and Glaucoma chattoni. The sequences surrounding the sites of transcription initiation and termination are highly conserved. The only extensive homologies of the NTS regions occur in five sets of dispersed repetitive sequences. Type I, II and III repeats in the 5' NTS are strongly conserved in sequence between Tetrahymena and Glaucoma in the case of the type I and III repeats, and in location relative to the transcription initiation site in the case of type I and II repeats. We identify two new repeat types, designated IV and V, in the 3' NTS. The sequence of type IV repeats, and the location relative to the transcription termination site of type IV and V repeats, are conserved. All five types of repeats are interspersed with nonconserved DNA sequences. These results suggest that the five repeat types in the 5' and 3' NTSs are important in rRNA gene function; the sequence organization, and the differing rates of divergence between species of the repeat types, provide strong evidence for their functional selection through the process of molecular coevolution.

Animals↗

PubMed1981

Two separate regions of the extrachromosomal ribosomal deoxyribonucleic acid of Tetrahymena thermophila enable autonomous replication of plasmids in Saccharomyces cerevisiae.

Plasmids containing the nontranscribed central and terminal, but not the coding, regions of the extrachromosomal ribosomal deoxyribonucleic acid (rDNA) of Tetrahymena thermophila are capable of autonomous replication in Saccharomyces cerevisiae. These plasmids transform S. cerevisiae at high frequency; transformants are unstable in the absence of selection, and plasmids identical to those used for transformation were isolated from the transformed yeast cells. One plasmid contains a 1.85-kilobase Tetrahymena DNA fragment which includes the origin of bidirectional replication of the extrachromosomal rDNA. The other region of Tetrahymena rDNA allowing autonomous replication of plasmids in S. cerevisiae is a 650-base pair, adenine plus thymine-rich segment from the rDNA terminus. Neither of these Tetrahymena fragments shares obvious sequence homology with the origin of replication of the S. cerevisiae 2-microns circle plasmid or with ars1, an S. cerevisiae chromosomal replicator.

Animals↗