摘要:

AbstractDuring mocronuclear development in the ciliated protozoanTetrahymena thermophila, sequence reorganization including sequence loss occurs. Addressing questions about the organization and nucleotide sequence of micronucleus limited regions can lead to insights about mechanisms of DNA rearrangements during macronuclear development as well as mechanisms for the maintenance of the stability of micronucleus‐limited sequence families. We have previously identified a moderately repetitive micronu‐cleus‐limited sequence family called X‐H (family members hybridize to an approximately 450 bpXbal‐HindIIIrestriction fragment), completely absent from macronuclear DNA. The first member of this family which we isolated is associated with terminal sequences characteristic of a Tel‐1 element, a putative micronuclear transposable element. Two additional family members have been isolated which are not closely associated with Tel‐1 terminal sequences. We have nucleotide sequence data for three cloned members of the X‐H family. This analysis has demonstrated that the longest cloned members of the X‐H family share a region of homology of approximately 2,400 bp and are highly conserved, differing only by small insertions or deletions of 100 bp or less. The sequences from one of the sequenced family members flanking the region of homology are themselves mostly micronucleus‐limited. ©

ISSN:0192-253X    

DOI:10.1002/dvg.1020130112

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractTelomeres, the G‐rich sequences found at the ends of eukaryotic chromosomes, ensure chromosome stability and prevent sequence loss from chromosome ends during DNA replication. During macronuclear development inTetrahymena, the chromosomes fragment into pieces ranging from 20 kb to 1,500 kb.Tetrahymenatelomerase, a ribonucleoprotein, adds telomeric (TTGGGG)nrepeats onto telomeres and onto the newly generated macronuclear DNA ends. We have investigated whether telomerase RNA levels increase during macronuclear development, since such an increase might be expected during chromosomal fragmentation. The steady‐state level of the telomerase RNA component was used to estimate the abundance of telomerase present in mating and nonmatingTetrahymena.Northern blot analysis revealed that in vegetatively growingTetrahymena, there were 18,000–40,000 copies of telomerase RNA per cell. In mating cultures, the levels of RNA increased 2‐to 5‐fold at 9–15 h, and 1.5‐ to 3.5‐fold in starved nonmating cultures. This increase in telomerase RNA paralleled telomerase activity, which also increased slightly in mating and starved nonmating cells. © 1992

ISSN:0192-253X    

DOI:10.1002/dvg.1020130113

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractCiliates possess nuclear dimorphism, i.e., they carry two structurally and functionally differentiated types of nuclei. The micronucleus and macronucleus serve as the germline and somatic nuclei, respectively, of the cell. The macronucleus differentiates from a mitotic sister of the micronucleus once per life cycle. Macronuclear differentiation is accompanied by a developmentally programmed set of DNA rearrangements, including chromosome fragmentation, telomere addition, and amplification. Given the diploidy of the MAC anlage, are both homologous copies of a chromosome processed and amplified equally and simultaneously in an individual differentiating MAC? We have approached this question for the case of the rDNA, exploiting previously identified DNA polymorphisms and the sensitivity of PCR. We determined allelic ratios in individual caryonide cells, i.e., the cells carrying the primary products of MAC differentiation, prior to the first division of the newly differentiated MAC. We observed stochastic variability in allelic ratios among caryonides that start with genetically identical heterozygous MACs. Either rDNA type can be in the majority. Appropriate controls make it unlikely that the ratios observed were significantly affected by variation in the assay itself. The variability may well result from the statistical variation associated with the relative timing of individual biochemical events initiating the processing and/or amplification of a few rDNA precursor molecules, presumably 4–8 at the most, in a MAC anlage. In addition to this stochastic variability, we observed a small but distinct bias in favor of the C3 rDNA. Thus the replication advantage of C3 relative to B rDNA in heterozygous MACs, previously detected during vegetative multiplication, may begin to be expressed during developmental amplification. We discuss the relevance of this stochastic developmental variability to classical genetic observations of Nanney and their collaborators on otherT. thermophilaloci. © 1992 Wiley‐Liss,

ISSN:0192-253X    

DOI:10.1002/dvg.1020130114

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

ISSN:0192-253X    

DOI:10.1002/dvg.1020130101

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY