[Palindromic and repetitive sequences in initiation sites of DNA replication in Tetrahymena]

DNA replication was induced in Tetrahymena pyriformis cells at the stationary phase of culture growth by a series of heat shocks. The selective nature of this DNA synthesis was established. DNA was uniformly labelled with [3H]thymidine during the exponential phase of culture growth and with [14C]thymidine during the stationary phase. The renaturation kinetics of DNA samples was investigated by measuring the relations of 14C/3H radioactivity in the renatured and denatured DNA fractions after their separation on hydroxyapatite. The results were analysed by means of a computer program based on the experimentally found renaturation curve of T. pyriformis macronuclear DNA. Under conditions described palindromic and repetitive (which are repeated about 1000 times) nucleotide sequences are intensively replicated. It is assumed that synchronous activation of replicons occurs as a consequence of induction. It may well be that replication starts in the central parts of comparatively long palindromes, which are identical or very similar in their nucleotide sequences.