Structural and Functional Characterization of an RNase HI Domain from the Bifunctional Protein Rv2228c from Mycobacterium tuberculosis

Abstract

ABSTRACT The open reading frame Rv2228c from Mycobacterium tuberculosis is predicted to encode a protein composed of two domains, each with individual functions, annotated through sequence similarity searches. The N-terminal domain is homologous with prokaryotic and eukaryotic RNase H domains and the C-terminal domain with α-ribazole phosphatase (CobC). The N-terminal domain of Rv2228c (Rv2228c/N) and the full-length protein were expressed as fusions with maltose binding protein (MBP). Rv2228c/N was shown to have RNase H activity with a hybrid RNA/DNA substrate as well as double-stranded RNase activity. The full-length protein was shown to have additional CobC activity. The crystal structure of the MBP-Rv2228c/N fusion protein was solved by molecular replacement and refined at 2.25-Å resolution ( R = 0.182; R free = 0.238). The protein is monomeric in solution but associates in the crystal to form a dimer. The Rv2228c/N domain has the classic RNase H fold and catalytic machinery but lacks several surface features that play important roles in the cleavage of RNA/DNA hybrids by other RNases H. The absence of either the basic protrusion of some RNases H or the hybrid binding domain of others appears to be compensated by the C-terminal CobC domain in full-length Rv2228c. The double-stranded-RNase activity of Rv2228c/N contrasts with classical RNases H and is attributed to the absence in Rv2228c/N of a key phosphate binding pocket.