Characterization and developmental expression of single-stranded telomericDNA-binding proteins from mung bean (Vigna radiata)

We have identified and characterized protein factors from mung bean (Vigna radiata) nuclear extracts that specifically bind the single-stranded G-rich telomeric DNA repeats. Nuclear extracts were prepared from three different types of plant tissue, radicle, hypocotyl, and root, in order to examine c hanges in the expression patterns of telomere-binding proteins during the d evelopment of mung bean. At least three types of specific complexes (A, B, and C) were detected by gel retardation assays with synthetic telomere and nuclear extract from radicle tissue, whereas the two major faster-migrating complexes (A and B) were formed with nuclear extracts from hypocotyl and r oot tissues. Gel retardation assays also revealed differences in relative a mount of each complex forming activity in radicle, hypocotyl, and root nucl ear extracts. These data suggest that the expression of telomere-binding pr oteins is developmentally regulated in plants, and that the factor involved in the formation of complex C may be required during the early stages of d evelopment. The binding factors have properties of proteins and are hence d esignated as mung bean G-rich telomere-binding proteins (MGBP). MGBPs bind DNA substrates with three or more single-stranded TTTAGGG repeats, while no ne of them show binding affinity to either double-stranded or single-strand ed C-rich telomeric DNA. These proteins have a lower affinity to human telo meric sequences than to plant telomeric sequences and do not exhibit a sign ificant binding activity to Tetrahymena telomeric sequence or mutated plant telomeric sequences, indicating that their binding activities are specific to plant telomere. Furthermore, RNase treatment of the nuclear extracts di d not affect the complex formation activities. This result indicates that t he single-stranded telomere-binding activities may be attributed to a simpl e protein but not a ribonucleoprotein. The ability of MGBPs to bind specifi cally the single-stranded TTTAGGG repeats may suggest their in vivo functio ns in the chromosome ends of plants.