HtrA, a Temperature- and Stationary Phase-Activated Protease Involved in Maturation of a Key Microbial Virulence Determinant, Facilitates Borrelia burgdorferiInfection in Mammalian Hosts

ABSTRACTHigh-temperature requirement protease A (HtrA) represents a family of serine proteases that play important roles in microbial biology. Unlike the genomes of most organisms, that of Borrelia burgdorferinotably encodes a single HtrA gene product, termed BbHtrA. Previous studies identified a few substrates of BbHtrA; however, their physiological relevance could not be ascertained, as targeted deletion of the gene has not been successful. Here we show that BbhtrAtranscripts are induced during spirochete growth either in the stationary phase or at elevated temperature. Successful generation of a BbhtrAdeletion mutant and restoration by genetic complementation suggest a nonessential role for this protease in microbial viability; however, its remarkable growth, morphological, and structural defects during cultivation at 37°C confirm a high-temperature requirement for protease activation and function. The BbhtrA-deficient spirochetes were unable to establish infection of mice, as evidenced by assessment of culture, PCR, and serology. We show that transcript abundance as well as proteolytic processing of a borrelial protein required for cell fission and infectivity, BB0323, is impaired in BbhtrAmutants grown at 37°C, which likely contributed to their inability to survive in a mammalian host. Together, these results demonstrate the physiological relevance of a unique temperature-regulated borrelial protease, BbHtrA, which further enlightens our knowledge of intriguing aspects of spirochete biology and infectivity.