| A small but growing number of bacteria and phages are known to contain linear, hairpin-ended genomes. The hairpin "protelomeres" are created by the action of a dedicated enzyme known as protelomerase that acts on a palindromic DNA target sequence. Phage protelomerases are typically longer than their bacterial counterparts and contain an additional far C-terminal region of limited sequence conservation. Studies of the protelomerase of the Klebsiella oxytoca phage phiKO2 have shown that although the far C-terminal region is not required to produce hairpin ends, truncation of the region has a drastic effect on enzyme kinetics. To date, no other studies have been reported on the far C-terminal region of this or any other protelomerase. We present the solution structures of the far C-terminal regions of two phage protelomerases. The regions form homologous, compact structures that adopt a fold similar to the canonical double-stranded RNA-binding domain and have been called the far C-terminal domains. Sequence alignment and secondary structure predictions show that all known and putative phage protelomerases contain C-terminal regions which will almost certainly form homologous domains. A sequence comparison of these proteins with all known protelomerases is presented, along with an analysis of the sequence and structure of proteins which adopt a similar fold. Based on structure homology and comparative sequence conservation of key binding regions, we propose that the domain belongs to the growing family of three stranded beta-sheet DNA-binding proteins that is a subclass of the double-stranded RNA-binding domain superfamily. |
