Mobile introns: Pathways and proteins

While the role of group I and group II intron-encoded proteins in homing has been well defined, the function of these proteins in intron dissemination to new sites remains the subject of intense study. These mobile introns, their intron-encoded proteins, and the mechanisms by which mobility occurs are the subject of this chapter. Although transition metals are not required for colicin DNase activity, it is likely that they play a stabilizing role related to the membrane translocation that must occur for colicin's biological function. These data lend credence to the idea that the HNH domain, like the GIY-YIG domain, is an endonu clease cassette that can become associated with other protein domains to form multifunctional proteins. The open reading frames (ORFs) specifying group II intron-encoded proteins, when present, are located in the loop region of the structural domain IV, with most of the coding sequence outside the intron catalytic core. Of the three activities of the group II intron-encoded proteins, the maturase domain is present in all known cases. Endonuclease activity of an intron-encoded protein was first shown for the yeast mtDNA introns aI1 and aI2. Group I and group II introns are self-splicing elements with wide genomic distribution, reflecting their dispersal through active mobility mechanisms. These two types of introns represent different ways in which selfish elements exploit functions that promote their invasiveness. Basic research into the structure and function of intron-encoded proteins and of the dynamics of mobility pathways is yielding a refined view of their modus operandi.