P
hag is the promoter of the
hag gene, which encodes the
flagellin protein . It has been shown to be one of the strongest constitutive promoters found in B. subtilis. In addition, the hag region of the genome, including flgM, is very well conserved. As such, the P
hag promoter offers strong and stable protein expression.
P
hag is inhibited by CsrA (“Carbon storage regulator A”), which binds at two sites of the promoter (denoted BS1 and BS2). Our designed BioBrick incorporates a single base-pair mutation in both these sites, which prevents CsrA binding.
Active P
hag requires binding of the SigD transcription factor at the -10 and -35 SigD sites. When flgM binds to SigD, it prevents binding to P
hag and therefore inhibits protein expression. The flgM gene is disrupted when pBSC
hagintegrates, thus achieving constitutive expression.