One of the genes up-regulated in theluxSmutant in both MHB and MEM-α iscj0982c, PFT�� manufacturer the product of which is a periplasmic binding protein specific for L-cysteine

and has been proposed to be part of an ABC transporter involved in cysteine uptake [55]. The increased expression of this gene may reflect the need of theluxSmutant to counteract the loss of homocysteine salvage by increasing cysteine uptake from the environment. There is also some homology between Cj1200 and the D-methionine-binding lipoprotein MetQ involved in import of D-methionine, although there is a second closer homologue elsewhere on theC. jejunichromosome. Expression of the putativemetFgene (Cj1202) was reduced in theluxSmutant grown in MEM-α and also in the stationary phase cells analysed by Heet al., 2008 [37]. MetF (methylenetetrahydrofolate reductase) catalyses the formation of 5-methyltetrahydrofolate, a cofactor required for providing a methyl group during the conversion of homocysteine Blasticidin S to methionine. The observed down-regulation ofmetFcould be the consequence of a regulatory mechanism that responds to the reduced availability of homocysteine. A very similar situation is present inS.

Typhimurium [20], where inactivation ofluxSreduced the expression ofmetE. InS. Typhimurium,metEexpression is positively regulated by MetR and homocysteine is known to considerably stimulate this activation [56,57]. Thus, reduced intracellular

homocysteine level in theS. TyphimuriumluxSmutant appears to be responsible for the reducedmetEexpression. A similar mechanism may have led to differential expression ofmetFinC. jejuniNCTC 11168luxS. Another example of a differentially regulated AMC gene ispfs(Cj0117). This gene, which was up-regulated in theluxSmutant in MHB both in logarithmic phase (this study) and in stationary phase [37], is required for the conversion of SAH (for the purpose Methocarbamol of detoxification and salvage of the resulting homocysteine and adenine moieties). The increase inpfstranscript levels could be the result of a regulatory mechanism responding to the concentration of AMC derivatives. Little is known about the regulation ofluxSandpfsin different bacteria, but at least inS. Typhimurium,pfsexpression depends on growth conditions and growth phase, whereasluxSis expressed constitutively [58]. The differential expression pattern of other genes is more difficult to explain and future work will need to address the regulatory mechanisms that respond to the intracellular changes associated with AMC disruption.