Acts both as a biotin--[acetyl-CoA-carboxylase] ligase and a biotin-operon repressor. In the presence of ATP, BirA activates biotin to form the BirA-biotinyl-5'-adenylate (BirA-bio-5'-AMP or holoBirA) complex. HoloBirA can either transfer the biotinyl moiety to the biotin carboxyl carrier protein (BCCP) subunit of acetyl-CoA carboxylase, or bind to the biotin operator site and inhibit transcription of the operon. ATP + biotin + L-lysyl-[protein] = AMP + diphosphate + H(+) + N(6)-biotinyl-L-lysyl-[protein] The switch between the enzymatic activity and the repressor activity is regulated by cellular demand for biotin. The switch occurs by swapping of protein interaction partners by holoBirA. In conditions of high biotin demand, holoBirA associates with apoBCCP to transfer biotin. In conditions of low biotin demand, holoBirA dimerizes, binds DNA and represses transcription of the biotin operon. Monomer in solution. Interacts with BCCP. Homodimerizes to bind DNA. Interaction with the corepressor bio-5'-AMP increases dimerization. Contains an N-terminal helix-turn-helix DNA-binding domain, connected via a linker to the central catalytic domain and the C-terminal domain, which plays roles in dimerization, catalytic function and DNA binding. The N-terminal domain is required for both ligase and repressor activities. It may orient the active site and thereby play an important role in enzymatic activity. Belongs to the biotin--protein ligase family.