Low molecular weight fluorescent probes with good photostability for imaging RNA-rich nucleolus and RNA in cytoplasm in living cells.

We have synthesized two low molecular weight organic molecules, PY and IN successfully, which selectively stain nucleolus and cytoplasm of living cells in 30 min, with a much lower uptake in the nucleus. Nucleic acids electrophoresis and digest test of ribonuclease indicate their markedly higher affinity for RNA, especially PY. Moreover their RNA localization in cells is further supported by digest test of ribonuclease, namely, the nucleolar fluorescence signal is distinctly lost upon treatment with RNase. And, the fact that live cells stained by PY and IN still possess physiological function can be confirmed: 1) MTT assay demonstrates that the mitochondria of cells stained remains its electron mediating ability, 2) Double assay of PY/IN and propidium iodide as well as trypan blue testing show that the membrane of cells stained still is intact. Importantly, compared with the only commercial RNA probe, SYTO RNA-Select, PY and IN exhibit much better photostability when continuously illuminated with 488 nm laser and mercury lamp. These results prove that PY and IN are very attractive staining reagents for visualizing RNA in living cells.