The targeting of Xcat2 mRNA to the germinal granules depends on a cis-acting germinal granule localization element within the 3 ' UTR

The germ cell lineage is specified by the germ plasm, which in Xenopus laev is contains putative determinants called germinal granules. The pathway thr ough which these structures form and how their components are assembled rem ain unclear. Using a combination of electron microscopy and in situ hybridi zation with the germinal granule-associated Xcat2 mRNA we demonstrated that the granules were derived from a branching network of granulofibrillar mat erial within the mitochondrial cloud. Targeting of Xcat2 mRNA to the germin al granules depended on a 164-nt 3'UTR germinal granule localization elemen t (GGLE; nt 631-795) that was distinct from the previously defined mitochon drial cloud localization element (MCLE; nt 403-630; Y. Zhou and M. L. King, 1996, Development 122, 2947-2953). This demonstrated that the Xcat 3'UTR c ontains a compound localization element consisting of a general element (MC LE) targeting the RNA to the mitochondrial cloud and a second element (GGLE ) responsible for targeting to the germinal granules within the cloud. The GGLE when fused to Xlsirt RNA was sufficient to target this nongranule mito chondrial cloud-associated RNA to the germinal granules. This is the first example of a localization element involved in targeting an mRNA to a specif ic subcellular target such as the germinal granules and suggests that cis-a cting elements on RNAs play an important role in the assembly of germinal g ranules and, therefore, the establishment of the germ cell lineage. (C) 200 0 Academic Press.