Abstract
The correct course of DNA replication is crucial to maintaining the integrity of the genome. Any abnormality in this process inevitably leads to replication stress (RS). Hydroxyurea (HU) is a replication stressor widely used to inhibit DNA biosynthesis by depleting the deoxyribonucleoside triphosphate (dNTP) pool. The aim of the study was to examine how the 24-, 48-, and 72 h exposures to 0.75 mM HU affect the localization of fibrillarin (FBL; a highly conserved nucleolar protein and the component of Cajal bodies) and the amount of rRNA transcripts (detected using 5-ethynyl uridine; 5-EU), in root meristem cells of Allium cepa. The consequence of prolonged RS was initially (after 24 h of incubation in HU) a 2-fold increase in 5-EU incorporation into the nucleolus, then (after 48- and 72 h incubations) followed by a gradual decrease in rRNA transcription to a level similar to that of the control. In interphase and in early prophase, both in the control material and during successive periods of incubation of root meristems in HU, the immunofluorescence of FBL accumulated in the fibrillar centers (FCs) of the nucleoli, in the dense fibrillar components (DFC), and in the granular components (GC). In some HU-treated metaphase cells, FBL was localized around the telomeres of the chromosomes, while in telophase, it was found in the fragmented chromosomes. In addition, an increase in the number of Cajal bodies (CBs) was observed during subsequent incubation periods with HU. After 48 and 72 h of treatment with HU, the number of CBs was found to be almost twice that observed in the control series. CBs disappeared in prophase and reappeared in interphase. These results suggest that depending on the duration of RS, changes in the level of rRNA transcription and in the abundance of CBs may correlate with the production of RNP and ribosome biogenesis.