A NOVEL DNA-BINDING DOMAIN IN THE SHRUNKEN INITIATOR-BINDING PROTEIN (IBP1)

South-western screening of lambda gt 11 expression library with a frag ment of the Shrunken promoter containing the initiator element resulte d in cloning of a novel maize gene. The encoded initiator-binding prot ein (IBP1) interacts at the transcription start site of the Shrunken p romoter. Analysis of the 680 amino acid (aa) long polypeptide revealed a novel bipartite DNA-binding domain at the carboxyl terminus. In its amino-terminal part, it is weakly related to Myb R-repeats but the fo llowing basic region is also essential for DNA binding. A region of si milarity to the conserved 2.1 and 2.2. motifs in bacterial sigma-facto rs is located close to the IBP1 amino terminus. Two putative nuclear l ocalization signals are compatible with the presence of antigenically related polypeptides in nuclear protein extracts. The IBP1 gene was ma pped to the long arm of chromosome 9 (9L095); a second highly related gene IBP2 is located on the short arm of chromosome 1 (1S014). Both ge nes encode proteins sharing 93% similarity and are transcribed with si milar activity in different plant organs. A small 82 nucleotide intron in the IBP2 transcript is found unspliced to a variable degree in dif ferent tissues. Translation of this incompletely processed transcript would result in a truncated amino-terminal polypeptide lacking the DNA -binding domain.