DNase I hypersensitivity of the terminal inverted repeat DNA sequences in the macronucleus of the ciliate Stylonychia mytilus.

Ciliates contain two types of nuclei: a diploid transcriptionally inactive micronucleus and a DNA-rich transcriptionally active macronucleus. After sexual reproduction of these cells the old macronucleus degenerates and a new macronucleus is formed by a micronuclear derivate. In hypotrichous ciliates (i.e., Stylonychiu, Oxytricha, k’uplotes) macronuclear development includes the formation of polytene chromosomes, elimination of >90% of the DNA and the specific fragmentation of the macronuclear genome into gene-sized DNA molecules with a size of -0.3-20 kilobases [ 1 -lo]. All of the macronuclear DNA molecules carry the same 20 basepairs terminal inverted repeat sequence which seems to be not only identical within the same species but in all hypotrichous ciliates examined [ 11-l 41; Swanton et al., personal communication; unpublished results). This terminal sequence is 5’-Cq&C4A4C4. These terminal sequences may be able to aggregate with each other forming multistranded DNA complexes [ 151. This observation together with the high sequence conservation made it interesting to examine its organization in the chromatin of the macronucleus. We first examined the DNAase I sensitivity of this sequence and compared it to that of bulk macronuclear DNA. Since most of the macronuclear DNA molecules probably represent structural genes [S-9,16] our study deals with the DNase I sensitivity of a DNA sequence adjacent to the coding region of eucaryotic genes.