Sequencing of the genomes of Mycobacterium tuberculosis H37Rv and Streptomyces coelicolor A3(2) identified putative genes for an NAD+-dependent DNA ligase. We have cloned both open reading frames and overexpressed the protein products in Escherichia coli. In vitro biochemical assays confirm that each of these proteins encodes a functional DNA ligase that uses NAD+ as its cofactor. Expression of either protein is able to complement E. coli GR501, which carries a temperature-sensitive mutation in ligA. Thus, in vitro and in vivo analyses confirm predictions that ligA genes from M. tuberculosis and S. coelicolor are NAD+-dependent DNA ligases.