Isolation, characterization and structure of subtilisin from a thermostable Bacillus subtilis isolate

A serine protease has been isolated and characterized from Bacillus subtilis, strain RT-5 (a thermostable soil isolate from the Tharparkar desert of Pakistan) able to grow at 55 degrees C. The primary structure was established by a combination of protein and DNA-sequence analyses. The amino-acid sequence, inhibition pattern and solubility properties identify the enzyme as a subtilisin. It has 43 amino-acid replacements toward subtilisin BPN' and as much as 83 replacements toward another subtilisin, confirming that strain variabilities are extensive between different subtilisin forms. However, the structure is identical to one of unknown functional properties deduced from DNA and is closely related to mesentericopeptidase but that homologue is not thermostable. From comparisons with that form and with subtilisin BPN', it is concluded that replacements of Ala --> Ser at positions 85 and 89, Ser --> Ala at position 88 and Asp or Ser --> Asn at position 259 may promote thermostability.