Supplementary Figure 2: Details on data reduction in the data analysis workflow.

The sample data is submitted into the ID filter pipeline. Here, a search is performed against the target-decoy version of a protein database containing the proteome of the species of interest as well as contaminant sequences. Peptides below a certain false discovery rate (FDR; default 1%) are considered as good matches and reported in an idXML file. Corresponding MS/MS spectra are removed from the mzML file. Next, the RNPxlXICFilter tool automatically screens whether the precursors of remaining MS/MS spectra also appear in the non-irradiated control. To this end, extracted ion chromatograms of those precursors are calculated in UV and control sample. MS/MS spectra of precursors appearing in both samples with comparable intensities, i.e. less than double in the UV sample, are not written into the output mzML file.