Replication-dependent and independent regulation of HMG expression during the cell cycle and conjugation in Tetrahymena

Abstract

Two abundant high-mobility-group (HMG)-like proteins, HMG B and HMG C, exist in the ciliated protozoan, Tetrahymena thermophila. Of these, HMG C is specific to transcriptionally active macronuclei, while HMG B is found in macronuclei and in transcriptionally inactive micronuclei [1]. Using Northern and in situ analyses, we show that the genes encoding HMG B and HMG C are not expressed uniformly throughout the vegetative cycle or during the sexual process, conjugation. Elevated expression of both genes is observed during macronuclear S phase of the vegetative cycle and during endoreplication of developing new macronuclei in later stages of conjugation. Interruption of any of these macronuclear DNA replications by aphidicolin leads to a rapid drop in the message levels of HMG B and HMG C. These results resemble what is typically observed for replication-dependent nucleosomal histones and differ from the apparent lack of cell cycle regulation observed for HMG genes in vertebrates. A specific-induction of HMG B mRNA is also observed early in conjugation and during this interval, inhibition of micronuclear DNA synthesis by aphidicolin does not affect the message level of HMG B. Thus, during conjugation, expression of HMG B shows both replication-dependent and independent regulation. Results similar to these with HMG B are obtained with histone H4II gene, a gene which is also expressed during micro- and macronuclear S phases during the vegetative cycle. These results demonstrate surprising complexity in the expression of HMG genes in Tetrahymena and lend support to the hypothesis that cell cycle regulation plays an important role in directing HMG-like proteins to the appropriate nucleus [2]. Interestingly, expression of neither HMG gene is perfectly synchronized with that of histone H4II gene during the developmental program suggesting that important differences exist between vegetatively growing (cell cycle control) and conjugating (developmental control) cells.