More on piRNA

Not so long ago I wrote about piRNA. After reading a bit more, there are some points I'd like to make:

- It would seem that piRNA (read this for background) are required for proper spermiogenesis.
- The argonaute family members associated with piRNA (Piwi, Miwi, Mili, Riwi ...) are only expressed in testes.
- The piRNA transcripts all come from specific loci in the genome.

It would seem like the piRNA may be processed fragments of a few transcripts that are transcribed from certain promoters within the genome. These piRNA don't seem to recognize retrotransposons or other "selfish genes" so it is unlikely that they serve the same role as rasiRNA (again see the old post for more details). Perhaps they are involved in translational silencing? (analogous to how siRNA and the RITS complex turns off gene expression by stimulating histone modifications that can spread localy within the genome.)

Ref:
Girard A, Sachidanandam R, Hannon GJ, Carmell MA.
A germline-specific class of small RNAs binds mammalian Piwi proteins.
Nature (06) 442:199-202

Grivna ST, Beyret E, Wang Z, Lin H.
A novel class of small RNAs in mouse spermatogenic cells.
Genes Dev (06) 20:1709-14

Aravin A, Gaidatzis D, Pfeffer S, Lagos-Quintana M, Landgraf P, Iovino N, Morris P, Brownstein MJ, Kuramochi-Miyagawa S, Nakano T, Chien M, Russo JJ, Ju J, Sheridan R, Sander C, Zavolan M, Tuschl T.
A novel class of small RNAs bind to MILI protein in mouse testes.
Nature (06) 442:203-7