title: A unique pattern of intrastrand anomalies in base composition of the DNA in hypotrichs.
abstract: The 50 non-coding bases immediately internal to the telomeric repeats in the two 5' ends of macronuclear DNA molecules of a group of hypotrichous ciliates are anomalous in composition, consisting of 61% purines and 39% pyrimidines, A>T (ratio of 44:32), and G>C (ratio of 17:7). These ratio imbalances violate parity rule 2, according to which A should equal T and G should equal C within a DNA strand and therefore pyrimidines should equal purines. The purine-rich and base ratio imbalances are in marked contrast to the rest of the non-coding parts of the molecules, which have the theoretically expected purine content of 50%, with A = T and G = C. The ORFs contain an average of 52% purines as a result of bias in codon usage. The 50 bases that flank the 5' ends of macronuclear sequences in micronuclear DNA (12 cases) consist of approximately 50% purines. Thus, the 50 bases in the 5' ends of macronuclear sequences in micronuclear DNA are islands of purine richness in which A>T and G>C. These islands may serve as signals for the excision of macronuclear molecules during macronuclear development. We have found no published reports of coding or non-coding native DNA with such anomalous base composition.
authors.affiliation: University of Colorado, Department of Molecular, Cellular and Developmental Biology, Boulder, CO 80309-0347, USA. prescotd@spot.colorado.edu
authors.forename: D M
authors.initials: DM
authors.lastname: Prescott
authors.forename: S J
authors.initials: SJ
authors.lastname: Dizick
mesh_terms: D000818:Animals; D001482:Base Composition; D016054:DNA, Protozoan / Q000737:chemistry / Q000235:genetics*; D016810:Hypotrichida / Q000235:genetics*; D017422:Sequence Analysis, DNA; D016615:Telomere / Q000235:genetics
chemical_list: D016054:DNA, Protozoan