Multiple, independently regulated, polyadenylated messages for histone H3 and H4 in Tetrahymena

Abstract

Heterologous probes for yeast H4 and H3 histone genes have been used to study the corresponding histone mRNAs in growing and starved Tetrahymena. Histone mRNAs in both physiological states are polyadenylated. Two types of H4 protein and two types of H3 protein have previously identified in Tetrahymena. Two size classes of H4 messages and three classes of H3 messages have been detected by northern analyses. Southern blot analysis indicate that the number of different kinds of H3 and H4 genes is the same or slightly greater than the number of different messages, suggesting that each message is derived from a different gene. Growing cells have -30 times more histone mRNA than starved cells, even though their total mRNA content is only 4 times greater. The relative abundance of different H4 and H3 messages in growing and starved cells is different, demonstrating that the different messages for a particular type of histone are regulated non-coordinately. In starved cells the presence of a single size class of H3 messages correlates with the preferential synthesis of a previously described macronuclear-specific H3 variant. The fraction of histone messages loaded in growing and starved cells is the same as for bulk mRNAs, and the relative concentrations of the multiple messages for H4 and H3 are the same in polysomal and total RNAs of each cell type. These observations suggest that histone synthesis in Tetrahymena is controlled largely at the level of message abundance, and that very little, if any, control occurs at the translational level.