Identification of Maize (Zea Mays L.) Genes Encoding Telomere Repeat DNA-Binding Proteins

In order to understand the molecular basis of meiotic telomere behavior, we have set out to identify genes that encode proteins with telomere repeat DNA-binding activity in maize (Zea mays L.), a model organism system. Using a combination of sequence similarity searches and DNA hybridization library screens we have isolated, sequenced, and characterized several full-length cDNA clones that resulted in the identification of two new gene families. These are the Single Myb histone (Smh) and the Terminal acidic SANT (Tacs) gene families. The Smh genes encode small (~32 kDa), basic proteins with a unique triple motif structure consisting of an N-terminal SANT/Myb-like domain, a centrally located linker histone H1 globular domain, and a coiled-coil dimerization domain near the C terminus. The SMH proteins appear to be unique to the plant kingdom and are encoded by small gene families in Arabidopsis, rice, and the PcMYB1 gene of parsley. The Smh1 gene is expressed in leaves and maps genetically to chromosome 8 (bin 8.05), with a duplicate locus on chromosome 3 (bin 3.09). A recombinant full-length SMH1, rSMH1, was found by band-shift assays to bind double-stranded oligonucleotide probes with at least two internal tandem copies of the maize telomere repeat, TTTAGGG. Point mutations in the telomere repeat residues reduced or abolished the binding, whereas rSMH1 bound nonspecifically to single-stranded DNA probes. We focused next on a second class of plant genes, which encode ~45 kDa proteins with a C-terminal acidic SANT (SWI3, ADA2, N-COR, and TFIIIB B') domain. A full-length cDNA from a juvenile leaf and shoot mRNA library was chosen for further analysis and named Terminal acidic SANT 1 (Tacs1). Polymorphic Tacs1 PCR products from genomic DNA map to the genetic bin 2.08. The Tacs1 gene structure and predicted protein sequence are nearly identical to those of the rice ANTHER INDEHISCENCE1 (AID1) gene of rice, indicating that maize Tacs1 and rice AID1 are homologous genes. Analysis of maize EST databases revealed that Tacs1 mRNA is found at relatively low levels, limited to very young reproductive organs, such as the primordia of ear and tassel shoots. The SANT domain is similar in sequence and structure to the DNA-binding domain (DBD) of Myb-related proteins, but the SANT domain exhibits an acidic surface chemistry that may impart a different function to this domain. The SANT domain of the maize TACS1 protein exhibits an acidic isoelectric point and a negative electrostatic surface potential. The discovery of these two plant gene families will help decipher the protein composition of plant telomeres and shed some light on telomere functions associated with some of these proteins.