A-factor-like autoregulators

A-factor, or (2S, 3R)-2-isocapryloyl-3-hydroxymethyl-γ-butirolactone, has been described by A.S. Khokhlov with coworkers and is one of the first studied autoregulators in prokaryotes. A-Factor structure has been confirmed by synthesis. It has been established that actinobacteria produce many substances with structural features closely related to the A-factor, regulating the development of Streptomyces griseus; particularly, they contain γ-bytirolactone, a hydroxymethyl group at position 2, and a fatty acid residue at position 3. These autoregulators are closely related not only in terms of their structure, but also their function, that is, regulation of processes of morphological differentiation, spore formation, and biosynthesis of secondary metabolites, including various antibiotics. This provides grounds for calling them A-factor-like regulators, or gamma-butyrolactones (GBLs), as they are often abbreviated. Structures of 21 natural autoregulators of the group isolated from representatives of eight streptomyces species have been established. Reference strains were used to demonstrate that A-factor regulators are typical of many species of the Actinomycetales order and are specific for these bacteria. They have been described in many species of Streptomyces,Actinomyces, Nocardia, Amycolatopsis, and Micromonospora. Autoregulators exhibit cross-effects with respect to reference strains of various species producing them. Presumably, biosynthesis of A-factor regulators is performed according to a common mechanism starting from fatty acid residues and glycerol as initial metabolites and involving iso-beta-ketoacid; the latter one is cyclized using a molecule of oxidized glycerol forming a nonsaturated gamma-lactone through decondensation at position 2 of oxidized glycerol, which is followed by reduction to A-factor. The first stage of biosynthesis is, supposedly, performed by the product of afsA gene; AfsA is the key enzyme in A-factor biosynthesis. AfsA protein homologues have been found in various streptomyces species. Molecular and genetics studies of A-factor-like autoregulators of S. griseus and some other streptomyces species allowed deciphering a regulatory cascade resulting in morphological differentiation and biosynthesis of secondary metabolites under the effect of nanomolar concentrations of the autoregulators. The action of the A-factor starts with its binding to the A-factor receptor protein (ArpA), which represses the promotor of the target gene. ArpA comprises two domains: N-terminal DNA-binding domain and the A-factor-binding C-terminal domain. ArpA protein binds to the adp4 gene, but DNA is depressed upon A-factor–ArpaA complex formation. This results in transcription of adpA gene encoding a transcription activator AdpA, the central regulator of A-factor regulatory cascade. AdpA amplifies the signal of A-factor, acting as a pleiotropic activator of transcription of at least 72 genes, particularly, the spore formation genes and genes of streptomycin biosynthesis in S. griseus. Altogether, genes activated by AdpA protein form the AdpA regulon. The AdpA-binding consensus DNA sequence has been established. According to their structure, the proteins can be grouped into a larger subfamily of the AraC/XylS family. Study of A-factor-like regulators is of topical interest for both theoretical and practical needs of antibiotic production development.