Activity of proline uptake in Escherichia coli CSH4 was inhibited in the presence of 1 M NaCl, while it was recovered if the cells were incubated at 30 degrees C for 1 h in a moderate salinity stress (MSS) solution which consists of Davis minimal medium with 5 mM praline and 0.5 M NaCl. Then, an attempt was made to examine whether MSS treatment is also effective on the activity restoration of proline uptake and growth under high salinity for E. coli CSH4 mutants with different combinations of proP, putA, putP, and proU which are related to the transport and metabolization of proline. After MSS treatment, proline uptake was vigorously occurred for the mutants with proline transporter gene prop but not for its deficient ones. For the expression of proline uptake activities of these mutant strains after MSS treatment, PO43- in MSS solution is more important than K+. No growth of strain CSH4 and its mutants without MSS treatment was observed, when cultured in high osmotic medium G (0.8 M NaCl) consisting of 1 mM glycine betaine and Davis minimal medium without potassium phosphate supplemented. After MSS treatment, however, mutant strains lacking prop showed sufficient growth in medium G. Cell growth of proP(+) strains was recognized if MSS treatment was performed in the absence of proline. In conclusion, growth of mutant strains under high-salinity medium G depended on their amount of proline accumulated during MSS treatment, in which K+ and PO43- might play a key role to guarantee their sufficient growth. (C) 2009, The Society for Biotechnology. Japan. All rights reserved.