Transcription of alpha-tubulin and histone H4 genes begins at the same point in the Physarum cell cycle.

Abstract

Abstract. In naturally synchronous plasmodia of Physarum polycephalum, both tubulin and histone gene transcription define periodic cell cycle-regulated events. Using a slot-blot hybridization assay and Northern blot analysis, we have demonstrated that a major peak of accumulation of both a-tubulin and histone H4 transcripts occurs in late G2 phase. Nuclear transcription assays indicate that both genes are transcriptionally activated at the same point in the cell cycle: mid G2 phase. While the rate of tubulin gene transcription drops sharply at the M/S-phase boundary, the rate of histone gene transcription remains high through most of S phase. We conclude that the cell cycle regulation of tubulin expression occurs primarily at the level of transcription, while histone regulation involves both transcriptional and posttranscriptional controls. It is possible that the periodic expression of both histone and tubulin genes is triggered by a common cell cycle regulatory mechanism.

Cell-cycle-regulated translation of histone mRNA in Physarum plasmodia

“…We have reported that both transcription of the histone genes and accumulation of histone mRNA begin with an event in early to mid-G2 phase, about 3 h before the S phase begins (1). The maximum accumulation of histone mRNA occurs at the end of the G2 phase, in the absence of significant DNA synthesis.…”

“…The maximum accumulation of histone mRNA occurs at the end of the G2 phase, in the absence of significant DNA synthesis. While histone gene expression is clearly cell cycle regulated in P. polycephalum, mRNA accumulation is not dependent on DNA replication (1). Transcription of these genes is directly regulated by a cell cycle control mechanism rather than occurring as an indirect response to increased demand for the histone proteins during the S phase (1).…”

“…The storage of inactive histone mRNA in oocyte nuclei presented an attractive hypothesis to explain messenger storage in P. polycephalum. To determine whether histone H4 mRNA is stored in the nucleus or cytoplasm, nuclear and whole-cell RNAs were isolated, and the proportion of histone H4 mRNA in each was examined by Northern blot analysis as described elsewhere (1). Neither histone H4 nor actin sequences could be detected in 15-,ug samples of nuclear RNA (Fig.…”

“…Our methods and probes are as described elsewhere (1). Actin mRNA served as a non-cell-cycle-regulated control.…”

“…To determine whether the histone mRNA stored in the late G2 phase is competent to be translated, we isolated plasmodial RNA both before and after entry into the S phase as described elsewhere (1). Samples (15 ,ug) were translated in wheat germ extracts by using protocols provided by the manufacturer (Amersham Corp., Arlington Heights, Ill.), and the labeled proteins were resolved as described above.…”