ISOLATION AND CHARACTERIZATION OF PIONEER1, A NOVEL CHLAMYDOMONAS TRANSPOSABLE ELEMENT

During the course of this study a novel family of Chlamydomonas mobile elements has been identified in natural isolate strain 224. The first member of this class to be characterized, a 2.8-kb element named Pion eer1, was trapped in an intron of the nitrate reductase structural gen e, NIT1. This element has been cloned and completely sequenced and fou nd to be unusual in structure. Pioneer elements are present in a very low-copy number of three per genome in strain 224. The copy number inc reased by one upon transposition of Pioneer1. Hybridization of Pioneer 1 to a variety of Chlamydomonas strains confirmed that this element di ffered from previously described Chlamydomonas transposons. It also in dicated that related elements are present in low-copy number in natura l isolate strains 356 and S1D2, but not in the most commonly used labo ratory strains 137c and 21 gr. For these reasons, members of the Pione er family might prove useful as insertional mutagens.