Construction and Characterization of a Mycobacterium tuberculosis Mutant Lacking the Alternate Sigma Factor Gene, sigF

Abstract

ABSTRACT The alternate RNA polymerase sigma factor gene, sigF , which is expressed in stationary phase and under stress conditions in vitro, has been deleted in the virulent CDC1551 strain of Mycobacterium tuberculosis . The growth rate of the Δ sigF mutant was identical to that of the isogenic wild-type strain in exponential phase, although in stationary phase the mutant achieved a higher density than the wild type. The mutant showed increased susceptibility to rifampin and rifapentine. Additionally, the Δ sigF mutant displayed diminished uptake of chenodeoxycholate, and this effect was reversed by complementation with a wild-type sigF gene. No differences in short-term intracellular growth between mutant and wild-type organisms within human monocytes were observed. Similarly, the organisms did not differ in their susceptibilities to lymphocyte-mediated inhibition of intracellular growth. However, mice infected with the Δ sigF mutant showed a median time to death of 246 days compared with 161 days for wild-type strain-infected animals ( P < 0.001). These data indicate that M. tuberculosis sigF is a nonessential alternate sigma factor both in axenic culture and for survival in macrophages in vitro. While the Δ sigF mutant produces a lethal infection of mice, it is less virulent than its wild-type counterpart by time-to-death analysis.