Genes For Regulating Other Genes

RNA "Genes" and the Regulation of RNA

mRNA is an indirect code bearer for the translation of protein, but there are more direct RNA-based coding mechanisms. One mechanism for posttranscriptional gene silencing (known in these acronym laden days as PTGS) is RNA interference (RNAi). Stretches of DNA that include antisense sequences that are complementary to parts of specific mRNA,are transcribed into RNA that folds into a double-stranded (dsRNA) form. As currently understood, the latter is recognized and cleaved by an RNase (RNA-degrading nuclease) enzyme in the Dicer gene family, releasing small interfering RNA (siRNA) fragments of 20-25 bp. The siRNA has antisense sequence to part of the target mRNA and together with a protein complex called RISC (RNA-induced silencing complex) binds the target, making it thus partly double-stranded so that it is degraded by RNases (Banerjee and Slack 2002; Cerutti 2003; Grosshans and Slack 2002; Hannon 2002; Sharp 1999; Tijsterman et al. 2002; Zamore 2002) (see Figure 7-2). There may be an intermediate step in which the dsRNA is amplified to many copies so the inhibition can be more effective in the cell. These chains of events are diverse within and between species and involve carrier and facilitator proteins coded by a gene family called Argonaute.

Another and related mechanism involves similar processing machinery to activate small temporal RNA (stRNA). The roughly 70-bp stRNA transcript self-hybridizes to form a stem-loop structure that is inactive until a short segment

Temporal Development RNAi stRNA precursor dsRNA

stRNA precursor single stranded stRNAs

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