DNA Replication-Dependent Histone H2A mRNA Expression in Pea Root Tips

Abstract

Histone H2A mRNA is selectively expressed in scattered subpopulations of cells in the pea (Pisum sativum) root apical meristem. To study whether this specific expression was associated with the cell cycle, a double-labeling technique was used to identify cells replicating DNA during S phase and those expressing H2A mRNA. Cells in S phase were detected by rH]thymidine incorporation and autoradiography, whereas cells containing H2A mRNA were identified by in situ hybridization using digoxigenin-labeled probes. Approximately 92% of the [3H]thymidine-labeled S-phase cells expressed H2A mRNA and 85% of cells that expressed H2A mRNA were in S phase. In root tissue located basal to the promeristem, synchronous co-located expression was observed in scattered packets of proliferating cells. Furthermore, neither H2A mRNA nor Sphase cells could be detected within the quiescent center or mature root cap. When DNA synthesis was inhibited with hydroxyurea, a commensurate and specific decrease in steady-state levels of H2A mRNA was found. We concludethat cell-specific expression of pea histone H2A mRNA is replication dependent and that H2A mRNA is transiently accumulated during a period of the cell cycle that mostly overlaps the S phase. We propose that the overlap between H2A expression and S phase could occur if H2A mRNA accumulation began in late G1 and abated in late S.Histone proteins are required for the packaging of eukaryotic DNA into chromatin and can be divided in three groups based on their characteristic modes of expression (Schiimperli, 1986). The largest group is cell-cycle-regulated and exhibits expression that is dependent on DNA replication. The second group, consisting of replacement variants, exhibits replication-independent expression and is expressed in nondividing cells of mature tissues. A third minor group, consisting of tissue-specific variants, is expressed in a replication-independent manner in unique tissues such as reproductive organs. Differences in genomic organization and gene structure mark each group. The regulation of replicationdependent histone expression has been extensively studied and reviewed (Hereford et al.,

Clustering of Microarray Data Reveals Transcript Patterns Associated with Somatic Embryogenesis in Soybean,

“…Transcript abundance of genes in set 1 peaks in the adaxial side at 7 d, decreases by one-half by 14 d, and is increasingly high in the adaxial side compared with the abaxial side overtime. Markers of the S-phase of the cell cycle such as histone H4 (Frank et al, 2000), histone H3 (Gown et al, 1996), and histone H2A (Tanimoto et al, 1993) suggest that genes in this set are involved in DNA replication. Their clustering with a cell division cycle 48 homolog (CDC48) suggests that DNA replication is concomitant with cytokinesis because CDC48 participates in the formation of spindle poles in proliferating tissue of plants (Feiler et al, 1995).…”

Constitutive Expression Exposes Functional Redundancy between the Arabidopsis Histone H2A Gene HTA1 and Other H2A Gene Family Members

“…In most organisms, core histone genes are expressed primarily during the S phase of the cell cycle (Tanimoto et al, 1993;Oswald et al, 1996;Downs et al, 2000). In addition, there are histones, called replacement histones, that are expressed constitutively in nondividing cells (Chaubet-Gigot et al, 2001).…”

Histone mRNAs Do Not Accumulate during S Phase of either Mitotic or Endoreduplicative Cycles in the ChordateOikopleura dioica

“…In mammals, the major basis for histone pre-mRNA regulation is cell cycle regulation of the SLBP (32). In plants and yeast, where replication-dependent histone transcripts are polyadenylated, coupling of DNA replication and histone synthesis is achieved mainly through transcriptional regulation (11,27,28).…”