Localization and Expression of mRNA for a Macronuclear-Specific Histone H2A Variant (hv1) during the Cell Cycle and Conjugation of Tetrahymena thermophila

Abstract

hv1 is a histone H2A variant found in the transcriptionally active Tetrahymena macronucleus but not in the transcriptionally inert micronucleus. This, along with a number of other lines of evidence, suggests that hv1 is associated with active genes. We have used a cDNA clone as a probe to study hv1 mRNA accumulation throughout the cell cycle and during conjugation. In situ hybridization to glutaraldehyde-fixed growing cells, whose position in the cell cycle was determined by size and morphology, showed that hv1 message is present throughout the cell cycle. The message was uniformly distributed in these vegetative cells. Compared with four other Tetrahymena histone genes studied to date (S. -M. Yu, S. Horowitz, and M. A. Gorovsky, Genes Dev., 1:683, 1987; M. Wu, C. D. Allis, and M. A. Gorovsky, Proc. Natl. Acad. Sci. USA 85:2205, 1988), hv1 mRNA is the only one that does not show a pattern of accumulation during the cell cycle that could explain the nuclear localization of its encoded protein. Thus, either hv1 or some molecule with which it associates contains a macronuclear-specific targeting sequence or there exists a cell cycle-regulated event that restricts its translation to the macronuclear S phase. In situ hybridization to conjugating cells revealed that hv1 message amounts increase just prior to macronuclear development and decline precipitously after the cells separate. The hv1 message showed no marked subcellular localization and is, therefore, unlikely to play a role in the cytoplasmic determination known to occur during macronuclear development.