All information based on an article by Deutscher (PMID: 19215777).
Cells adjust their ribosome concentration to growth rate such that fast growing cells may have 10-fold more ribosomes than slow growing ones. As cells encounter conditions of nutritional deprivation and their growth rate slows or ceases, excess ribosomes may be degraded.
A major question surrounding the degradation of ribosomes under stress conditions is how the process is initiated. The simplest explanation that would allow ribosome degradation to be built into the system when cells enter slow or no growth would be to have the actual substrates be the 30S and 50S ribosome subunits.
RNases that take part in this process:
1) Many RNases had not yet been identified so the enzymology of the process was necessarily incomplete.
2) First attack on ribosome by an endoribonuclease (RNase I), followed by exonuclease action (PMID: 4358625) - the exonucleases involved: PNPase and RNase II (PMID: 4358625) or a different hydrolytic enzyme (PMID: 320188).
3) RNase I is not present in an active form inside cells - in vitro studies have shown that RNases E and G are able to carry out cleavages similar to RNase I (Zundel and Deutscher, unpublished observations).
4) One report has suggested that the endoribonuclease, RNase LS, may participate in the degradation of 23S rRNA as a fragment of this molecule accumulated in its absence.
5) With regard to the exoribonucleases that participate in rRNA degradation, it is now known that only PNPase, in association with an RNA helicase, or RNase R are able to digest highly structured RNAs (PMID: 15664199).
6) Studies in vitro have confirmed that PNPase and RNase R can act on ribosome subunits (Zundel and Deutscher, unpublished observations).