Acquisition of PrtS in Streptococcus thermophilus is not enough in certain strains to achieve rapid milk acidification

Abstract

The acquisition of prtS by Streptococcus thermophilus strains allowed hydrolysis of caseins into peptides and then to increase their growth in milk. This leads to faster milk acidification, which is important in dairy industry. However, some strains harboring the same allele of prtS present different acidification rates, which could be explained by a difference in the regulation of prtS expression. We chose two strains with the same allele of prtS (including the same promoter region): one, PB302, is with high acidification rate while the other, PB18O, is without. They exhibited similar growth in M17, but not in milk, where PB302 showed better growth. The expression of prtS and activity of PrtS were lower in PB18O, in the two media tested. We demonstrated that other genes known to be involved in carbon and nitrogen metabolism were overexpressed in PB302. Interestingly, these genes were overexpressed in milk compared to M17. Nearly all these genes possessed a putative CodY-box in their promoter region. Taken together, difference of gene expression detected in PB302 between milk (low-peptide medium) and M17 (rich-peptide medium) and presence of a putative CodY-box is a feature of the transcriptional pattern of CodY-regulated genes. Altogether, our results propose that acquisition of prtS is not enough in certain strains to achieve rapid milk acidification. High transcriptional level of dtpT, amiF, ilvC, ilvB, bcaT, livJ, ackA, codY, and prtS in fast acidifying strain suggests that this transcriptional pattern could be required for fast milk acidification in Streptococcus thermophilus.

Characteristics of the Proteolytic Enzymes Produced by Lactic Acid Bacteria

“…lactis cells, and the inhibitory potency is modulated in an intracellular environment rich in amino acids such as isoleucine, leucine, and valine [ 89 , 90 ]. While regulating the proteolytic system, the CodY repressor acts as a blocking agent that binds to the CodY operator sequence (AATTTTCWGAAAATT) and prevents RNA polymerase from accessing the promoter sites of individual components of the proteolytic system [ 4 , 91 , 92 , 93 ]. To sum up, when the intracellular concentration of the amino acids isoleucine, leucine, and valine increases, several of these amino acids attach to the allosteric site of the CodY repressor by changing the conformation of this protein in a such way that it allows it to bind to the operator in the DNA; consequently, this combination blocks the transcription of genes responsible for enzyme synthesis.…”

Comparative Genomics of Streptococcus thermophilus Support Important Traits Concerning the Evolution, Biology and Technological Properties of the Species

“…However, the sole presence of prtS is not sufficient for the rapid milk acidification by S. thermophilus. Milk acidification seems to be a complex phenotypic trait, which involves the overexpression of several genes (Galia et al, 2016). Furthermore, it was demonstrated that S. thermophilus strains, irrespective of the prtS +/− status, may present cellassociated extracellular peptidase activities.…”

Proteolytic System of Streptococcus thermophilus

“…Despite the previous information, not all the S. thermophilus strains possess PrtS, e.g., strain CNRZ1066 lacks prtS gene and, in contrast, strain LMD-9 displays strong proteolytic activity due to the presence and expression of prtS gene [36]. It has been shown that for high proteolytic activity, the presence of a putative CodY-box is necessary in their promoter region, which is a feature of the transcriptional pattern of CodY-regulated genes that are able to bind promoters of 14 genes belonging to the proteolytic system, including PrtS [37]. It has been shown that the interaction between S. thermophilus, with or without PrtS, and Lactobacillus, especially Lb.…”