Gene Clusters During the Development and Replication of the Macronucleus in Tetrahymena thermophila

The organization of the 5s rRNA genes in the MACronuclear genome of Tetrahymena thermophila was examined during MAC development and replication. The 5s genes are arranged in several tandem arrays of alternating transcribed and spacer sequences in both MICronucleus and MAC. The number of EcoRI fragments bearing 5s gene clusters is similar in MIC and MAC. Most fragments occur in both the MIC and newly formed MAC genomes, a few being MIC-limited and a few MAC-limited. The same rearrangements are seen in the MACs of all four caryonides of a mating pair, and most rearrangements are seen in the newly formed MACs of different inbred strains. During replication of the MAC about half the fragments bearing 5s gene clusters disappear in different cell lines, and new fragments containing 5s genes appear. These fragments differ in size from those present in the MIC or newly formed MAC. These alterations occur in the MACs of all strains except strain B, which is more resistant to vegetative rearrangement. The losses and gains of fragments occur during clonal propagation of cell lines. The process begins by 35 fissions following conjugation, but once an alteration occurs, it is stably propagated. Clonal variation occurs with respect to which losses and gains occur, although a nonrandom distribution is seen among cell clones. We conclude that the alterations in MAC fragment size occur at two stages in the life cycle of Tetrahymena. The first stage occurs during conjugation, when the MAC develops from the MIC. The second stage becomes manifest during vegetative growth, when DNA replication occurs in the MAC and daughter molecules are distributed “amitotically ” to daughter nuclei. The two-stage character to MAC alterations for the 5s genes is interpreted in terms of the two steps previously described for MAC differentiation: determination and phenotypic assortment. Possible molecular mechanisms are also discussed.