Unusual intragenic suppression of an IFT52 gene disruption links hypoxia to the intraflagellar transport inTetrahymena thermophila

IFT52 protein is a conserved intraflagellar transport protein (a part of the IFT complex B) that is essential for assembly and maintenance of cilia. Tetrahymena null mutants with an insertion of a neo gene cassette into the IFT52 gene undergo frequent suppressions that lead to conditional assembly of cilia only under hypoxic conditions (Brown et al. 2003). Here we show that these conditional suppressions are intragenic and occur by a novel mechanism. First, the non-native (bacterial) portion of the DNA sequence of the neo cassette is deleted during the process of genome rearrangement that occurs in the developing macronucleus of conjugating Tetrahymena . Next, the residual sequences of the neo cassette (of Tetrahymena origin) within the IFT52 mRNA are recognized as multiple introns and undergo splicing, leading to a restoration of the translational frame of IFT52 . The resulting hypoxia-dependent IFT52 protein contains an insertion of 43 new amino acids that replace 7 original amino acids. Taken together with a study in Chlamydomonas reinhardtii showing a hypoxia-dependence of another IFT subunit mutant, IFT46, (Hou et al. 2007), our observations generalize that defective IFT complex subunits can regain functionality under hypoxia.