Nucleotide sequence divergence among DNA fractions of different syngens of Tetrahymena pyriformis

Abstract

The magnitude of the differences in base sequence of DNA fractions derived from different syngens of the ciliated protozoan Tetrahymena pyriformis was investigated. Each DNA was fractionated into unique and repeated sequences by hydroxylapatite chromatography, and the fractions were tested by in vitro molecular hybridization techniques. The amount of hybrid formed and the thermal stability of the hybrid molecules were examined at different incubation temperatures (50 and 65 C) for unique sequences and at 50 C for repeated sequences. The extent of the reactions involving either unique or repeated sequences was nearly complete when the two DNAs compared were derived from the same syngen. Moreover, intrasyngenic hybrids formed at 50 C (and 65 C for unique sequences) exhibit a high degree of thermal stability. In contrast, the extent of the reactions involving sequences derived from different syngens was low, as expected from the effect of mismatching on rate of reassociation, and intersyngenic hybrids formed at 50 C have low thermal stability. The reaction of unique sequences is further reduced at 65 C and the intersyngenic hybrids formed have a higher thermal stability than those formed at 50 C. The degree to which thermal stability is lowered was then used to estimate the percentage of mispaired bases. The average divergence of unique sequences between syngens is large and of the magnitude found for rodent DNAs from different genera or for Drosophila DNAs from nonsibling species. The repeated sequence fraction may contain more than one component and may be more conserved than the unique sequence fraction.