E. Virginia Merriam
Professor Emeritus of Biology | Loyola Marymount University
Los Angeles, CA, UNITED STATES
Education (2)
University of Washington: Ph.D., Biochemistry 1966
Elizabethtown College: B.S., Chemistry 1962
Areas of Expertise (1)
The Cytogenetics of Tetrahymena
Links (2)
Articles (2)
The Highly Conserved Family of Tetrahymena thermophila Chromosome Breakage Elements Contains an Invariant 10-Base-Pair Core
Eukaryotic CellEileen P Hamilton, Sondra Williamson, Sandra Dunn, Esther Virginia Merriam, Cindy Lin, Linh Vong, Jessica Russell-Colantonio, Eduardo Orias
2006 As a typical ciliate, Tetrahymena thermophila is a unicellular eukaryote that exhibits nuclear dimorphism: each cell contains a diploid, germ line micronucleus (MICN) and a polyploid, somatic macronucleus (MACN). During conjugation, when a new MACN differentiates from a mitotic descendant of the diploid fertilization nucleus, the five MICN chromosomes are site-specifically fragmented into 250 to 300 MACN chromosomes. The classic chromosome breakage sequence (CBS) is a 15-bp element (TAAACCAACCTCTTT) reported to be necessary and sufficient for chromosome breakage. To determine whether a CBS is present at every site of chromosome fragmentation and to assess the range of sequence variation tolerated, 31 CBSs were isolated without preconception as to the sequence of the chromosome breakage element. Additional CBS-related sequences were identified in the whole-genome sequence by their similarities to the classic CBS. Forty CBS elements behaved as authentic chromosome breakage sites. The CBS nucleotide sequence is more diverse than previously thought: nearly half of the CBS elements identified by unbiased methods have a variant of the classic CBS. Only an internal 10-bp core is completely conserved, but the entire 15-bp chromosome breakage sequence shows significant sequence conservation. Our results suggest that any one member of the CBS family provides a necessary and sufficient cis element for chromosome breakage. No chromosome breakage element totally unrelated to the classic CBS element was found; such elements, if they exist at all, must be rare.
Genome-Wide Characterization of Tetrahymena thermophila Chromosome Breakage Sites. I. Cloning and Identification of Functional Sites
GeneticsEileen P Hamilton, Peter John Bruns, Cindy Lin, Esther Virginia Merriam, Eduardo Orias, Linh Vong, Donna M Cassidy-Hanley
2005 The chromosomes of the macronuclear (expressed) genome of Tetrahymena thermophila are generated by developmental fragmentation of the five micronuclear (germline) chromosomes. This fragmentation is site specific and directed by a conserved 15-bp chromosome breakage sequence (Cbs element). This article reports the construction of a library enriched for chromosome breakage junctions and the development of a successful scheme for the genome-wide isolation and characterization of functional Cbs junctions. Twenty-three new Cbs junctions were characterized and each was assigned to a specific micronuclear chromosome or chromosome arm. Two distinct previously unreported variant chromosome breakage sequences were found, each in two or more functional Cbs elements. Analysis of natural Cbs junctions confirmed that microheterogeneity in the macronuclear telomere addition site is associated with chromosome fragmentation. The physical and genetic characterization of these functional chromosome breakage junctions is reported in the accompanying article in this issue. The whole-genome shotgun sequencing and auto-assembly phase of the Tetrahymena Genome Initiative has recently been completed at The Institute for Genome Research (TIGR). By providing unique sequence from the natural ends of macronuclear chromosomes, Cbs junctions characterized in the work reported here will serve as useful sequence tags for relating macro- and micronuclear genetic, physical, and sequence maps.
