ac4C RNAs recognizes UVA and UVC-induced DNA lesions
Local laser microirradiation showed that ac4C RNAs recruit to microirradiated chromatin immediately after genome injury. The ac4c RNA signal weakens 20-40 min post-irradiation (Fig. 1). In non-irradiated cells, we have observed that ac4C RNA occupies a compartment of nucleoli (Fig. 1), which was additionally verified by dual immunolabelling showing in parallel ac4 RNA and fibrillarin, a main component of nucleoli (Fig. 2A, B). The fibrillarin-positive region of nucleoli of non-irradiated cells was dense on ac4C RNA, and inhibitor of RNA polymerase I, actinomycin D, caused the so-called crescent-like morphology of the fibrillarin-positive region of nucleoli. Also, these crescents colocalized with ac4C RNAs (Fig. 2A). Ac4C RNAs recognized UVA-induced DNA lesions also when the cells were treated by actinomycin D (Fig. 2B).
When the cells were as the whole cell population irradiated by UVC lamp, we found a high density of ac4C RNA in the nucleoplasm of the cells exposed to UVC and analyzed 20 minutes post-irradiation. Cells analyzed 90-120 minutes after irradiation were characterized by ac4C RNA reorganization into well-visible, tiny foci (Fig. 3A-E). In such UVC-irradiated cells, we quantified ac4C RNA distribution in the whole nuclear content (Fig. 3B). As can be seen in Figures 3A and 3E, in non-irradiated cells, the most significant amount of ac4C RNA was concentrated in nucleoli; on the other hand, after UVC irradiation, the highest fluorescence intensity we observed in the nucleoplasm (3A-E). The ratio of the relative fluorescence intensities (FI) in the nucleus to the rest of FI of ac4C RNA in the nucleus was 2.42 (median) in the control cells. For irradiated samples, at intervals 5 min, 20 min, 50 min, and 120 min after irradiation, the FI range was from 0.44 to 0.94 (on average). The difference in relative intensities was highly statistically significant (p≤0.0001) (Fig. 3E). The most marked changes in the nucleoplasm were detected 5 minutes after irradiation, then the amount of ac4C RNA decreased. Differences in absolute fluorescence intensity within the nucleus were statistically significant in all compared groups (p≤0.0001); (Fig. 3B, 3C). Interestingly, the total FI of fluorescently stained ac4C RNA in the nucleoli was significant after irradiation, and an increase in ac4C RNA inside the nucleoli was also 120 min after irradiation, but the trend in the nucleoplasm was the opposite (Fig. 3D, 3E). These results suggest that UV radiation increases the acetylation of N4-cytidine in nucleoplasmic RNAs.
Accumulation of ac4C RNA in DNA lesions is PARP dependent
We have addressed the question if a pronounced appearance of ac4C at DNA lesions is PARP-dependent. We treated the cells with a PARP inhibitor, olaparib. In non-irradiated cells, we have observed a high density of ac4 RNA in nucleoli, 10 min after UVA irradiation, a high level of ac4 RNAs was detected in the nucleoplasm, while the cells treated by PARP inhibitor were characterized by the identical distribution profile fo ac4C RNA as in control non-irradiated cells (Fig. 4A). PARP inhibitor olaparib also prevents the recruitment of ac4C RNA to UVA-microirradiated chromatin; thus, these data show that accumulation of ac4C RNAs to locally-induced DNA is PARP-dependent (Fig. 4B).
Recruitment of ac4C at UVA-damaged chromatin is not dependent on the function of NAT10 acetyltransferase
It is well-known that RNA cytidine acetyltransferase NAT10 is responsible for the installation of N4-acetylcytidine (ac4C) on mRNAs, 18S rRNA, and tRNAs [13, 14]. It was observed that NAT10 is responsible for the formation of ac4C at position 1842 in 18S rRNA [13]. Based on this information, we analyzed if recruitment of ac4C RNA to DNA lesions is NAT10 depend. In this case, we found that NAT10 is not recruited to microirradiated chromatin; thus, it seems likely that ac4C installation on RNA at UV-induced DNA lesions is not mediated via NAT10 acetyltransferase (Fig. 5).
Here, we also compare ac4C RNA accumulation at microirradiated chromatin with m6A RNA appearance at DNA lesions, as we studied recently [17]. We found distinct recruitment kinetics for ac4C RNAs and m6A RNAs at DNA lesions. In general, a peak of accumulation is identical for both ac4C RNAs and m6A RNAs, but a high level of ac4C RNAs at microirradiated chromatin remains longer, up to 30-45 min postirradiation, in comparison to m6A RNAs (Fig. 6).
An increase in ac4C RNA level after UV irradiation is identical in all phases of the interphase.
For such experiments, we used HeLa-Fucci cells expressing RFP-Cdt1 in the G1 phase and GFP-tagged geminin in the G2 phase of the cell cycle [20]. In G1, S, and G2 cell cycle phases, we observed an identical increase in ac4C RNA level when the whole cell population was irradiated by UV light and compared with non-irradiated cells (Fig. 7A, B).