Transsulfuration is the main reaction in microorganisms (30) and plants (11) to incorporate sulfur into a four-carbon amino acid, L-homocysteine, which is produced from L-cystathionine (CTT) through a reaction catalyzed by cystathionine ␤-lyase (EC 4.4.1.8). These organisms first synthesize L-cysteine with O-acetyl-L-serine (OAS) and sulfide and subsequently synthesize CTT with L-cysteine and L-homoserine by the catalysis of CTT ␥-synthase (EC 4.2.99.9). This enzyme reacts with an activated form of L-homoserine: O-acetyl-Lhomoserine (OAH) (fungi and some bacteria) (4,15,24), O-succinyl-L-homoserine (enteric and some other bacteria) (11, 13, 28), or O-phosphoryl-L-homoserine (plants) (9,26). Thus, many organisms obtain homocysteine by incorporating sulfur that is first assimilated into cysteine, via CTT. However, a few microorganisms synthesize homocysteine directly through a replacement reaction of OAH with sulfide that is catalyzed by OAH sulfhydrylase (EC 4.2.99.10) (6,23,34).Little is known about sulfur metabolism and the related enzymes of bacteria living under extreme conditions such as alkaline pH or high temperature. Some archaea have been reported to synthesize cysteine from methionine through reversed transsulfuration from homocysteine to cysteine (36), but an OAS sulfhydrylase (EC 4.2.99.8) has been purified from an archaeon and characterized in detail, suggesting that the enzyme is functional as a cysteine synthase (1). Thus, two groups of archaea can be distinguished with respect to cysteine synthesis. This is supported by the result of genome analysis (16) showing that some archaea have genes homologous to the OAS sulfhydrylase gene of enteric bacteria, while others do not. Recently, we have found that an alkaliphilic bacterium produces a very large amount of OAS sulfhydrylase, the specific activity of which is very low compared with that of other microorganisms (31). Its extreme stability to heat and alkaline pHs is also notable. We have recently found activities of CTT ␥-synthase, CTT ␤-lyase, and OAH sulfhydrylase in a cell extract of an extremely thermophilic bacterium, Thermus thermophilus HB8, in addition to a very high OAS sulfhydrylase activity (35). On the other hand, Kosuge et al. (17,18) have reported that T. thermophilus HB27 synthesizes homocysteine through direct sulfhydrylation of OAH and that transsulfuration is not func-* Corresponding author. Mailing address: