IN-VIVO MOBILITY OF A GROUP-I TWINTRON IN NUCLEAR RIBOSOMAL DNA OF THE MYXOMYCETE DIDYMIUM-IRIDIS

DiSSU1 is an optional group I twintron present in the nuclear extrachr omosomal ribosomal DNA of the myxomycete Didymium iridis, DISSU1 appea rs to be complex both in structure and function. At the RNA level it h as a twin-ribozyme organization composed of two group I ribozymes with different functions, separated by an open reading frame. Here, we sho w that DISSU1 is mobile when haploid intron-containing and intron-less amoebae are mated, The mobility process is fast, being completed in 5 -10 nuclear cycles after mating in the developing zygote and plasmodia , Analyses of progeny from genetic crosses confirm intron mobility, Di SSU1 is the first example of a mobile group I twintron, The intron-enc oded protein was expressed in Escherichia coil and found to be an endo nuclease, I-Dirl, that cleaves an intron-less ribosomal DNA allele at the intron-insertion site, and is probably involved in intron homing, The endonuclease I-Dirl seems to be a rare example of a protein that i s expressed from a ribozyme-processed RNA polymerase I transcript in v ivo.