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

Abstract

In Physarum polycephalum, histone mRNA begins to accumulate 3 h before the S phase. We show that histone synthesis was limited to the S phase and that the mRNA that accumulated in the G2-phase cytoplasm could be translated in vitro. Thus, recruitment of stored mature histone mRNA did not occur until the S phase began.Histone gene expression has proven to be cell cycle regulated in virtually every eucaryote studied. Namely, synthesis of the core histones is limited to the S phase in organisms ranging from yeasts to humans. Generally, an increase in the level of histone mRNA parallels the peak of histone protein synthesis, although many species store pools of inactive maternal messenger in the oocyte nucleus for recruitment later during embryogenesis (for a review, see reference 7). Here, we report a unique exception to this rule.These experiments with mitotically synchronous plasmodia of the primitive myxomycete Physarum polycephalum show that histone mRNA is regularly stored during the G2 phase of its mitotic cycle. Since the P. polycephalum cell cycle has no Gi phase, its S phase begins at the end of mitosis. 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. 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).This note addresses the storage and activity of the G2-phase histone mRNA. The G2-phase accumulation of histone mRNA serves a useful purpose; it provides a dowry of mRNA to meet the maximum demand for new histone proteins that occurs at the beginning of the S phase. While preliminary studies of histone protein synthesis in P. polycephalum show that histone synthesis peaks in the S phase, the investigators failed to study histone synthesis in the last hour of the G2 phase (9). This left open the possibility that a dowry of histone proteins might accumulate before the S phase begins. The data presented below show that this did not occur; histone synthesis was limited to the S phase. Wilhelm et al. (10) reported an apparent shortening of histone H4 mRNA at the beginning of the S phase. Since this might reflect a change in the primary structure of the mRNA needed for its efficient translation, we compared the translatability of G2-phase and S-phase mRNAs in wheat germ extracts. Our data indicate that the efficiencies of translation were comparable. In addition, we show that the * Corresponding author.histone mRNA was stored in the G2-phase cytoplasm rather than in the nucleus. Therefore, we infer that during the G2 phase, mature histone mRNA is stored in the cytoplasm as a tr...

Cell-cycle-dependent translation of histone mRNAs is the key control point for regulation of histone biosynthesis in Leishmania infantum

“…phase, in the absence of DNA synthesis. The histone mRNAs are stored in the cytoplasm as translationally inactive RNPs until the beginning of the next S phase [32,33]. Nevertheless, the translational control is a part of the multi-level regulation of histone gene expression that this organism possesses, whereas in Leishmania the control of histone expression occurs only at the translational level.…”

Histone synthesis in Leishmania infantum is tightly linked to DNA replication by a translational control

“…A known example of this type of regulation is histone expression in Xenopus oocytes : translationally inactive histone mRNAs are stored in the frog oocytes, and translation is activated at oocyte maturation [28]. Also, translation regulation of histone mRNA has been described in Physarum polycephalum [29,30]. In this plasmodium, although histone mRNA begins to accumulate 3 h before the S phase, histone synthesis is limited to the S phase.…”

Detection of point mutations with a modified ligase chain reaction (Gap-LCR)

“…Multiple rounds of denaturation, annealing and ligation result in the exponential amplification of the target DNA ( Fig. IA) (13)(14)(15)(16)(17)(18). Targets that differ by a single base pair are discriminated, since a mismatch at the ligation joint severely reduces the efficiency of ligation (12)(13)(14)19).…”