A 16 KDA PROTEIN FAMILY OVEREXPRESSED BY STREPTOCOCCUS-THERMOPHILUS PB18 IN ACID ENVIRONMENTS

The one- and two-dimensional protein patterns of Streptococcus thermop hilus PB18 in the exponential and stationary phases of growth were ana lysed. One- dimensional SDS-PAGE showed that a 16 kDa protein was over expressed in stationary phase as well as 2 h after an acid shock, and that it was not expressed when the bacteria reached the stationary pha se in medium with limiting lactose concentrations (5 or 10 gl(-1)), in which the ph (5.5) was not as acid as in control cultures (ph 4.7, la ctose 20 gl(-1)). The results support the idea that this protein is ex pressed in response to the acidic environment and not in response to t he growth phase. Two-dimensional PAGE showed that nine proteins were e xpressed only during the exponential phase and ten others only during the stationary phase. The 16 kDa band seen in one-dimensional SDS-PAGE corresponded to a 16 kDa protein family observed on two-dimensional S DS-PAGE/IEF gels, whose expression was increased 8.5-fold when the ext racellular ph reached a critical value below 5.0. The N-terminal seque nces of proteins from two spots on the two-dimensional gels (members o f the 16 kDa family) were determined and found to be identical. The ph ysiological role of this protein family has not yet been elucidated.