Micronucleus-specific DNA sequences in an amicronucleate mutant of Tetrahymena

DNA from the amicronucleate Tetrahymena cell line BI3840 was probed for DNA sequences which are limited to the micronucleus in wild-type cells. Four micronucleus-specific DNA sequences were not detectable in DNA from the amicronucleate cell line. Two of the six micronucleus-specific DNA sequences tested hybridized to DNA from amicronucleate cells. Both the number of fragments homologous to these sequences and the intensity of hybridization were reduced in the DNA from the amicronucleate cells relative to DNA from a wild-type cell line, indicating that less than one micronucleus equivalent of the micronucleus-specific DNA sequences was retained in the amicronucleate cell line. Thus many micronucleus-specific DNA sequences were eliminated from the developing macronucleus of BI3840 as they are in wild-type cells, but in at least two cases the elimination was incomplete. In situ hybridization suggested that the DNA sequences which are limited to the micronucleus in wild-type cells are present in the macronucleus of the amicronucleate cell line. Southern blots of DNA from the amicronucleate cell line were also probed with DNA sequences which are retained in the macronucleus. At least two types of genome rearrangements occurred in the BI3840 macronucleus as they do in wild-type cells. No spurious rearrangements were observed.