DNA REPLICATION-DEPENDENT HISTONE H2A MESSENGER-RNA EXPRESSION IN PEAROOT-TIPS

Histone H2A mRNA is selectively expressed in scattered subpopulations of cells in the pea (Pisum sativum) root apical meristem. To study whe ther this specific expression was associated with the cell cycle, a do uble-labeling technique was used to identify cells replicating DNA dur ing S phase and those expressing H2A mRNA. Cells in S phase were detec ted by [H-3]thymidine incorporation and autoradiography, whereas cells containing H2A mRNA were identified by in situ hybridization using di goxigenin-labeled probes. Approximately 92% of the [H-3]thymidine-labe led S-phase cells expressed H2A mRNA and 85% of cells that expressed H 2A mRNA were in S phase. In root tissue located basal to the promerist em, synchronous co-located expression was observed in scattered packet s of proliferating cells. Furthermore, neither H2A mRNA nor S-phase ce lls 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 con clude that cell-specific expression of pea histone H2A mRNA is replica tion dependent and that H2A mRNA is transiently accumulated during a p eriod of the cell cycle that mostly overlaps the S phase. We propose t hat the overlap between H2A expression and S phase could occur if H2A mRNA accumulation began in late G1 and abated in late S.