Abstract
A novel proteinase, PrtR, produced by the human vaginal isolate
Lactobacillus rhamnosus
strain BGT10 was identified and genetically characterized. The
prtR
gene and flanking regions were cloned and sequenced. The deduced amino acid sequence of PrtR shares characteristics that are common for other cell envelope proteinases (CEPs) characterized to date, but in contrast to the other cell surface subtilisin-like serine proteinases, it has a smaller and somewhat different B domain and lacks the helix domain, and the anchor domain has a rare sorting signal sequence. Furthermore, PrtR lacks the insert domain, which otherwise is situated inside the catalytic serine protease domain of all CEPs, and has a different cell wall spacer (W) domain similar to that of the cell surface antigen I and II polypeptides expressed by oral and vaginal streptococci. Moreover, the PrtR W domain exhibits significant sequence homology to the consensus sequence that has been shown to be the hallmark of human intestinal mucin protein. According to its α
S1
- and β-casein cleavage efficacy, PrtR is an efficient proteinase at pH 6.5 and is distributed throughout all
L. rhamnosus
strains tested. Proteinase extracts of the BGT10 strain obtained with Ca
2+
-free buffer at pH 6.5 were proteolytically active. The
prtR
promoter-like sequence was determined, and the minimal promoter region was defined by use of
prtR-gusA
operon fusions. The
prtR
expression is Casitone dependent, emphasizing that nitrogen depletion elevates its transcription. This is in correlation with the catalytic activity of the PrtR proteinase.
