Analysis of the proteolytic system of Streptococcus thermophilus strains CS5, CS9, CS18 and CS20

Streptococcus thermophilus is a common milk starter, and the ability to hydrolyse proteins is an important property. The proteolytic systems of S. thermophilus strains CS5, CS9, CS18 and CS20 have been investigated according to their genomes. The growth and proteolytic abilities of these strains have been analysed in combination with gene expressions of both the proteolytic system and the two-component systems (TCSs) of the specified strains. When the strains are grown in milk, prtS is a key determinant in the fermentation rate of PrtS⁺ strains (CS5, CS18 and CS20), but other genes are activated in the PrtS⁻ strain CS9. The proteolytic system is regulated by the global transcriptional regulatory factor CodY, especially in the strain CS9. Furthermore, the expression of most genes involved in the proteolytic system and the TCSs were positively correlated with proteolytic activity of strains. The relationship between TCSs and proteolytic gene expression should be investigated in the future.