408
2012 ; reviewed in Lai and King 2013 ). In fact, the onset of transcription in PGCs
occurs some 10 or more hours after it is initiated in somatic cells at the mid-blastula
transition (MBT). The delay allows PGCs to avoid inappropriately responding to sig-
nals patterning the early embryo and initiated by the growth factor Vg1 and the tran-
scription factor VegT (reviewed in Heasman 2006 ). At the same time, PGCs have their
own unique gene program that is initiated during mid- gastrulation and early neurula
stages (Venkatarama et al. 2010 ). Genes transcribed at this time likely support a migra-
tion program and preserve the potential for meiosis and totipotency. Recent evidence
suggests that Nanos and Pumilio may be the lead players in translational repression
while Dead-end and Xdazl in translational activation in the Xenopus germ line.
Virtually all characterized mRNAs within the germ plasm are sequestered in a
translationally repressed state throughout oogenesis (Houston and King 2000b;
Kloc et al. 2001 ; 2002 ; Voronina et al. 2011 ; Schisa 2012 ) (Table 8.2). Nanos RNA
is repressed not only by being sequestered in germinal granules, but by steric hin-
drance via secondary structure just 4 nt downstream of the AUG site (Luo et al.
2011 ). What activates the translation of these mRNAs is important to understand.
Several candidate RNA-binding proteins with ATPase activity have been implicated
including Vasa, DeadSouth, and Dnd (Liu and Collodi 2010 ; Lasko 2013 ). Among
the first RNAs to be translated in pPGCs is nanos where Dnd appears to play a criti-
cal role relieving repression at the pre-initiation step (Luo 2011 ). Nanos is a broadly
conserved protein found in species as divergent as sponges and humans (Lai et al.
2011 ). The only known function for Nanos is as a translational repressor in partner-
ship with the RNA-binding protein Pumilio (Lai et al. 2011 , 2012 ). Pumilio recog-
nizes very specific nucleotide binding sites (PBE: UGUA(N)AUA). Pumilio
provides the selectivity for repression by recruiting Nanos to only those RNAs.
VegT and other vegetally localized RNAs contain PBEs in their 3′ UTR required for
translational repression (Lai et al. 2012 ; Lai and King 2013 ). In nanos mutants, the
endoderm specific transcription factors and downstream targets of VegT, Xsox17,
and Bix4, are now expressed in PGCs. Furthermore, the delay in transcription is lost
and PGCs initiate transcription at the same time as somatic cells (Lai et al. 2012 ).
PGCs lacking Nanos subsequently undergo apoptosis. Interestingly, nanos mutants
in Drosophila and C. elegans also display premature transcription and loss of the
germ line (Schaner et al. 2003 ; Deshpande et al. 2005 ). How can translational
repression by Nanos/Pumilio in PGCs be related to the transient genome-wide sup-
pression of mRNA transcription in these cells at the MBT?
Recently, a link between the translation and accumulation of maternal transcrip-
tion factors, including VegT, has been found to be critical to the normal onset of the
MBT (Skirkanich et al. 2011 ). In zebrafish, pluripotency factors Oct91 and Sox
were also found to be important in this regard (Harvey et al. 2013 ; Lee et al. 2013 ).
The working model that has emerged from these studies suggests that first genomic
DNA must be released from a repressed state. Such an event is correlated with the
nuclear to cytoplasmic ratio and depletion of a repressor (reviewed in Langley et al.
2014 ). Cell cycle regulation therefore, is one component, driving changes in the
N/C ratio. One target of nanos/Pumilio repression, apparently conserved across spe-
cies, is cyclin B1. Cyclin B1, in a concentration dependent fashion, regulates the
T. Aguero et al.