496
10.2.1 Elimination of Maternal Small RNAs
Small RNA pathways utilize small noncoding RNAs as sequence-specific guides
for mRNA degradation and/or translational repression (Ghildiyal and Zamore
2009 ). Vertebrates utilize three distinct small RNA pathways: RNA interference
(RNAi), microRNA (miRNA), and PIWI-associated RNA (piRNA) pathways
(Fig. 10.6). Molecular mechanisms of RNAi, miRNA, and piRNA pathways have
been described in detail elsewhere (Jinek and Doudna 2009 ; Kim et al. 2009 ;
Nejepinska et al. 2012a; Siomi et al. 2011 ; Chekulaeva and Filipowicz 2009 ).
Below, we provide only basic characterization of these pathways while focusing on
maternal small RNA functionality and turnover during OET.
10.2.1.1 microRNAs
miRNAs are genome-encoded posttranscriptional regulators of gene expression.
Nuclear miRNA precursor transcripts are preprocessed, transported to the cyto-
plasm, and cleaved by RNase III Dicer to produce ~22-nucleotide-long miRNAs,
which are loaded on Argonaute (AGO) proteins, which mediate silencing effects
(reviewed in Kim et al. 2009 ). Animal miRNAs typically base-pair imperfectly
with 3′UTRs of cognate mRNAs and cause their translational repression and/or
degradation. The silencing mode depends on the degree of complementarity and
Fig. 10.6 Schematic depiction of vertebrate RNA silencing pathways and their key components
P. Svoboda et al.