2-Methyl-4-carboxy,5-hydroxy-3,4,5,6-tetrahydropyrimidine (THP(A) or hydroxyectoine) and 2-methyl,4-carboxy-3,4,5,6-tetrahydropyrimidine (THP(B) or ectoine) are now recognized as ubiquitous bacterial osmoprotectants. To evaluate the impact of tetrahydropyrimidine derivatives (THPs) on protein-DNA interaction and on restriction-modification systems, we have examined their effect on the cleavage of plasmid DNA by 10 type II restriction endonucleases. THP(A) completely arrested the cleavage of plasmid and bacteriophage DNA by EcoRI endonuclease at 0.4 mM and the oligonucleotide (d(CGCGAATTCGCG)) 2 at about 4.0 mM. THP(B) was 10-fold less effective than THP(A), whereas for betaine and proline, a notable inhibition was observed only at 100 mM. Similar effects of THP(A) were observed for all tested restriction endonucleases, except for SmaI and PvuII, which were inhibited only partially at 50 mM THP(A). No effect of THP(A) on the activity of DNase I, RNase A, and Taq DNA polymerase was noticed. Gelshift assays showed that THP(A) inhibited the EcoRI-(d-(CGCGAATTCGCG)) 2 complex formation, whereas facilitated diffusion of EcoRI along the DNA was not affected. Methylation of the carboxy group significantly decreased the activity of THPs, suggesting that their zwitterionic character is essential for the inhibition effect. Possible mechanisms of inhibition, the role of THPs in the modulation of the protein-DNA interaction, and the in vivo relevance of the observed phenomena are discussed.Two tetrahydropyrimidine derivatives identified in Streptomyces bacteria (1-3), one a previously unknown metabolite, THP(A), 1 and the other previously identified (as ectoine) in halophilic bacteria (4), THP(B), are now recognized as widely spread osmoprotectants within the bacterial world (5). The role and activities of THPs are of special interest as they represent a limited group of osmoprotectants that are synthesized de novo, in the bacterial cell, in contrast to those transported from the medium (6). Their synthesis in a number of Streptomyces strains as a response to increased salinity and elevated temperature was recently described (7). THPs are small molecules, highly soluble in water and neutral at physiological pH. NMR and x-ray crystallography data show that THPs are zwitterionic molecules with a delocalized charge in the NCN group (Fig. 1) and form the half-chair conformation (8).More information has been accumulated lately on THPs activity in living cells. It was found that exogenously provided ectoine (THP(B)) could reverse growth inhibition, caused by osmotic stress, in Escherichia coli (9), Corynebacterium glutamicum (10), and the soil bacterium Rhizobium meliloti (11). We demonstrated that exogenously provided THP(A), like THP(B), reversed inhibition of E. coli growth by osmotic stress, and moreover, both THP(A) and THP(B) could stimulate growth of E. coli at an elevated temperature (43°C) (7). Recently cloned genes for ectoine synthesis from Halomonas elongata (12) and from Marinococcus halophilus (13) were demonstrated ...