Identification of a Mycobacterium tuberculosis gene that enhances mycobacterial survival in macrophages

Intracellular survival plays a central role in the pathogenesis of Mycobact erium tuberculosis, To identify M. tuberculosis genes required for intracel lular survival within macrophages, an M. tuberculosis H37Rv plasmid library tvas constructed by using the shuttle vector pOLYG. This plasmid library m as electroporated into Mycobacterium smegmatis 1-2c, and the transformants were used to infect the human macrophage-like cell line U-937, Because M. s megmatis does not readily survive within macrophages, any increased intrace llular survival is likely due to cloned nl: tuberculosis H37Rv DNA, After s ix sequential passages of nil. smegmatis transformants through U-937 cells, one clone (p69) was enriched more than 70% as determined by both restricti on enzyme and PCR analyses. p69 demonstrated significantly enhanced surviva l compared to that of the vector control, ranging from 2.4- to 5.3-fold at both 24 and 48 h after infection. DNA sequence analysis revealed three open reading frames (ORFs) in the insert of p69. ORF2 (1.2 kb) was the only one which contained a putative promoter region and a ribosome-binding site. De letion analysis of the p69 insert DNA showed that disruption of ORF2 result ed in complete loss of the enhanced intracellular survival phenotype, This gene was named the enhanced intracellular survival (eis) gene. By using an internal region of eis as a probe for Southern analysis, eis was found in t he genomic DNA of various ill. tuberculosis strains and of Mycobacterium bo vis BCG but not in that of ill, smegmatis or 10 other nonpathogenic mycobac terial species. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic a nalysis showed that all M. smegmatis eis-containing constructs expressed a unique protein of 42 kDa, the predicted size of Eis, The expression of this 42-kDa protein directly correlated to the enhanced survival of nl. smegmat is p69 in U-937 cells. These results suggest a possible role for eis and it s protein product in the intracellular survival of M. tuberculosis.