A Mycobacterium tuberculosis operon encoding ESAT-6 and a novel low-molecular-mass culture filtrate protein (CFP-10)

The early secreted antigenic target 6 kDa protein (ESAT-6) is a potent T-cell protein antigen synthesized by Mycobacterium tuberculosis. Its corresponding gene (esat-6) is located in RD1, a 10 kb DNA region deleted in the attenuated tuberculosis vaccine strain Mycobacterium bovis BCC. The promoter region of M. tuberculosis esat-6 was cloned and characterized. A new gene, designated Ihp and cotranscribed with esat-6 was identified. Moreover, computer searches in the M. tuberculosis genome identified 13 genes related to the Ihp/esat-6 operon, defining a novel gene family. The transcription initiation sites of the Ihp/esat-6 operon were mapped using M. tuberculosis RNA. The corresponding promoter signals were not recognized in Mycobacterium smegmatis, in which transcription of Ihp/esat-6 is initiated at different locations. The M. tuberculosis Ihp gene product was identified as CFP-10, a low-molecular-mass protein found in the short-term culture filtrate. These results show that the genes encoding CFP-10 and ESAT-6 are transcribed together in M. tuberculosis and that both code for small exported proteins.