We analyzed the role of posttranscriptional mechanisms in the regulation of histone gene expression in Saccharomyces cerevisiae. The rapid drop in histone RNA levels associated with the inhibition of ongoing DNA replication was postulated to be due to posttranscriptional degradation of histone transcripts. However, in analyzing the sequences required for this response, we showed that the coupling of histone RNA levels to DNA replication was due mostly, if not entirely, to transcriptional regulatory mechanisms. Furthermore, deletions which removed the negative, cell cycle control sequences from the histone promoter also uncoupled histone transcription from DNA replication. We propose that the arrest of DNA synthesis prematurely activates the regulatory pathway used in the normal cell cycle to repress transcription. Although posttranscriptional regulation did not appear to play a significant role in coupling histone RNA levels to DNA replication, it did affect the levels of histone RNA in the cell cycle. Posttranscriptional regulation could apparently restore much of the periodicity of histone RNA accumulation in cells which constitutively transcribed the histone genes. Unlike transcriptional regulation, periodic posttranscriptional regulation appears to operate on a clock which is independent of events in the mitotic DNA cycle. Posttranscriptional recognition of histone RNA must require either sequences in the 3' end of the RNA or an intact three-dimensional structure since H2A- and H2B-lacZ fusion transcripts, containing only 5' histone sequences, were insensitive to posttranscriptional controls.
The phenotype of a “Cdc2 kinase target site-deficient” mutant of oncoprotein 18 reveals a role of this protein in cell cycle control.
