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.