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The Rv0948c gene from Mycobacterium tuberculosis H37Rv encodes a 90 amino acid protein as the natural gene product with chorismate mutase (CM) activity. The protein, 90-MtCM, exhibits Michaelis–Menten kinetics with a kcat of 5.5 ± 0.2 s−1 and a Km of 1500 ± 100 μM at 37 °C and pH 7.5. The 2.0 Å X-ray structure shows that 90-MtCM is an all α-helical homodimer (Protein Data Bank ID: 2QBV) with the topology of Escherichia coli CM (EcCM), and that both protomers contribute to each catalytic site. Superimposition onto the structure of EcCM and the sequence alignment shows that the C-terminus helix 3 is shortened. The absence of two residues in the active site of 90-MtCM corresponding to Ser84 and Gln88 of EcCM appears to be one reason for the low kcat. Hence, 90-MtCM belongs to a subfamily of α-helical AroQ CMs termed AroQδ. The CM gene (y2828) from Yersinia pestis encodes a 186 amino acid protein with an N-terminal signal peptide that directs the protein to the periplasm. The mature protein, *YpCM, exhibits Michaelis–Menten kinetics with a kcat of 70 ± 5 s−1 and Km of 500 ± 50 μM at 37 °C and pH 7.5. The 2.1 Å X-ray structure shows that *YpCM is an all α-helical protein, and functions as a homodimer, and that each protomer has an independent catalytic unit (Protein Data Bank ID: 2GBB). *YpCM belongs to the AroQγ class of CMs, and is similar to the secreted CM (Rv1885c, *MtCM) from M. tuberculosis.