Ribosomal RNA associates with mitotic chromosomes and is passed to progeny.
Previous work in HeLa cells revealed that pre-rRNA associates with chromosomes during mitosis and segregates to each daughter cell during cytokinesis [28]. We reasoned that a similar mechanism could mediate rRNA transfer from Drosophila neural progenitors to their progeny. To test this possibility, we fed EU for 16 hours and imaged tagged rRNA along with phosphorylated histone H3, a marker of mitotic chromosomes, and Miranda, an asymmetrically localized protein that is briefly present in newly formed GMCs [29]. As described for HeLa cells, tagged rRNA overlapped closely with mitotic chromosomes, including chromosomes at the metaphase plate (Figure 3A ) and chromosomes inherited by newly formed GMCs (Figure 3B ).
To further test if rRNA is passed from progenitors to neurons, we performed a 6-hour EU “pulse” followed by an 18 hour “chase” with unmodified uridine. As described above, the majority of tagged rRNA was concentrated in progenitor cells at the end of the 6-hour feeding, but some newly born neurons (adjacent to progenitors) were positively labeled (Figure 3C ). Following the 18-hour chase, the rRNA signal was transferred from progenitors to recently born neurons. The transferred rRNA signal was strong throughout the cell, indicating localization to the nucleus (site of initial ribosome assembly) and cytoplasm (site of final ribosome maturation and mature ribosomes). These results support the rRNA inheritance model. Very little, if any, rRNA decay is expected during the 18-hour chase. We used larval neuroblast-specific EC-tagging pulse-chase experiments to measure RNA half-lives and detected no rRNA decay during a 12-hour chase (the longest chase timepoint tested, data not shown). Previous work has also shown that Drosophila rRNA is extremely stable; rRNA produced in embryos lasts into larval stages with a half-life between 48 and 115 hours depending on growth conditions [30]. The high stability of rRNA supports our conclusion that the tagged RNA detected in neurons at the end of the chase is intact inherited rRNA.