Evolutionary conservation and DNA binding properties of the Ssh7 proteins fromSulfolobus shibatae

The thermoacidophilic archaeonSulfolobus shibatae synthesizes a large amount of the 7-ku DNA binding proteins known as Ssh7. Our hybridization experiments showed that two Ssh7-encoding genes existed in the genome of S.shibatae. These two genes, designatedssh7a andssh7b, have been cloned, sequenced and expressed inEscherichia coli. The two Ssh7 proteins differ only at three amino acid positions. In addition, thecis-regulatory sequences of thessh7a andssh7b genes are highly conserved. These results suggest the presence of a selective pressure to maintain not only the sequence but also the expression of the two genes. We have also found that there are two genes encoding the 7-ku protein inSulfolobus solfataricus. Based on this and other studies, we suggest that the gene encoding the 7-ku protein underwent duplication before the separation ofSulfolobus species. Binding of native Ssh7 and recombinant (r)Ssh7 to short duplex DNA fragments was analyzed by electrophoretic mobility shift assays. Both native and recombinant forms of the protein behaved in a similar fashion in the assays, suggesting that the interaction of Ssh7 with DNA is not affected either by specific lysine methylation found in the native Ssh7 proteins or by the difference between the two Ssh7 isomers in amino acid sequence. Our data show that Ssh7 binds duplex DNA fragments with a binding size of ∼ 6.6 base pairs and an apparent dissociation constant of (0.7–1.0) × 10^-7 mol/L under the assay conditions employed in the present study. In addition, Ssh7 binds more tightly to negatively supercoiled DNA than to linear or relaxed DNA.