Preferential binding of fd gene 5 protein to tetraplex nucleic acid structures

The gene 5 protein of filamentous bacteriophage fd is a single-stranded DNA -binding protein that binds non-specifically to all single-stranded nucleic acid sequences, but in addition is capable of specific binding to the sequ ence d(GT(5)G(4)CT(4)C) and the RNA equivalent r(GU(5)G(4)CU(4)C), the latt er interaction being important for translational repression. We show that t his sequence preference arises from the formation of a tetraplex structure held together by a central block of G-quartets, the structure of which pers ists in the complex with gene 5 protein. Binding of gene 5 protein to the t etraplex leads to formation of a similar to 170 kDa nucleoprotein complex c onsisting of four oligonucleotide strands and eight gene 5 protein dimers, with a radius of gyration of 45 Angstrom and an overall maximum dimension o f 120-130 Angstrom. A model of the complex is presented that is consistent with the data obtained. It is proposed that the G-quartet may act as a nucl eation site for binding gene 5 protein to adjacent single-stranded regions, suggesting a novel mechanism for translational repression. (C) 2000 Academ ic Press.