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In the presence of alanine, AldR, which belongs to the Lrp/AsnC family of transcriptional regulators and regulatesald encoding alanine dehydrogenase inMycobacterium smegmatis , changes its quaternary structure from a homodimer to an octamer with an open-ring conformation. Four AldR-binding sites (O2, O1, O4, and O3) with a consensus sequence of GA/T-N2 -NWW/WWN-N2 -A/TC were identified upstream of theM. smegmatis ald gene by means of DNase I footprinting analysis. O2, O1, and O4 are required for the induction ofald expression by alanine, while O3 is directly involved in the repression ofald expression. In addition to O3, both O1 and O4 are also necessary for full repression ofald expression in the absence of alanine, due to cooperative binding of AldR dimers to O1, O4, and O3. Binding of a molecule of the AldR octamer to theald control region was demonstrated to require two AldR-binding sites separated by three helical turns between their centers and one additional binding site that is in phase with the two AldR-binding sites. The cooperative binding of AldR dimers to DNA requires three AldR-binding sites that are aligned with a periodicity of three helical turns. ThealdR gene is negatively autoregulated independently of alanine. Comparative analysis ofald expression ofM. smegmatis andMycobacterium tuberculosis in conjunction with sequence analysis of bothald control regions led us to suggest that the expression of theald genes in both mycobacterial species is regulated by the same mechanism.IMPORTANCE In mycobacteria, alanine dehydrogenase (Ald) is the enzyme required both to utilize alanine as a nitrogen source and to grow under hypoxic conditions by maintaining the redox state of the NADH/NAD+ pool. Expression of theald gene was reported to be regulated by the AldR regulator that belongs to the Lrp/AsnC (feast/famine) family, but the underlying mechanism was unknown. This study revealed the regulation mechanism ofald inMycobacterium smegmatis andMycobacterium tuberculosis . Furthermore, a generalized arrangement pattern ofcis -acting regulatory sites for Lrp/AsnC (feast/famine) family regulators is suggested in this study.

journal of bacteriology

Regulation Mechanism of the <i>ald</i> Gene Encoding Alanine Dehydrogenase in Mycobacterium smegmatis and Mycobacterium tuberculosis by the Lrp/AsnC Family Regulator AldR

Regulation Mechanism of the ald Gene Encoding Alanine Dehydrogenase in Mycobacterium smegmatis and Mycobacterium tuberculosis by the Lrp/AsnC Family Regulator AldR