Divergence towards a dead end? Cleavage of the divergent domains of ribosomal RNA in apoptosis

In several cases of apoptotic death the large ribosomal subunit 28S rRNA is specifically cleaved. The cleavages appear at specific sites within those domains of the rRNA molecule that have shown exceptional high divergence in evolution (D domains). The cleavages accompany rather than precede apoptosis, and there is a positive, but not complete, correlation between rRNA cleavage and internucleosomal DNA fragmentation. Most cell types studied so far show two alternative cleavage pathways that are mutually exclusive. Cleavage can either start in the D8 domain with secondary cuts within a subdomain of D2 (D2c), or in the D2 domain with subsequent excision of the D2c subdomain. The latter pathway is of particular interest since D2 (unlike D8) is normally inaccessible for RNase attack. That apoptosis specifically affects the ribosomal divergent domains suggests that these domains, which make up roughly 25% of total cellular RNA, might have evolved to serve functions related to apoptosis. Future studies will be directed to test the hypothesis that rRNA fragmentation may be part of an apoptotic program directed against the elimination of illegitimate (viral?) polynucleotides.