In vivo immunogold labeling confirms large-scale chromatin folding motifs.

The difficulty in localizing specific cellular proteins by immuno-electron microscopy techniques limits applications of electron microscopy to cell biology. We found that in vivo immunogold labeling improves epitope accessibility, ultrastructural preservation and three-dimensional visualization, and allows correlated light and electron microscopy. We detected large-scale chromatin folding motifs within intact interphase nuclei of CHO cells and visualized the ultrastructure of DNA replication 'factories' labeled with GFP-proliferating cell nuclear antigen (PCNA).