Abstract
Transcriptional control of carbon source preferences by
Acinetobacter
sp. strain ADP1 was assessed with a
pobA
::
lacZ
fusion during growth on alternative substrates. The
pobA
-encoded enzyme catalyzes the first step in the degradation of 4-hydroxybenzoate, a compound consumed rapidly as a sole carbon source. If additional aromatic carbon sources are available, 4-hydroxybenzoate consumption is inhibited by unknown mechanisms. As reported here, during growth on aromatic substrates,
pobA
was not expressed despite the presence of 4-hydroxybenzoate, an inducer that normally causes the PobR regulator to activate
pobA
transcription. Growth on organic acids such as succinate, fumarate, and acetate allowed higher levels of
pobA
expression. In each case,
pobA
expression increased at the end of the exponential growth phase. Complex transcriptional regulation controlled 4-hydroxybenzoate catabolism in multisubstrate environments. Additional studies focused on the wild-type preference for benzoate consumption prior to 4-hydroxybenzoate consumption. These compounds are degraded via the catechol and protocatechuate branches of the β-ketoadipate pathway, respectively. Here, mutants were characterized that degraded benzoate and 4-hydroxybenzoate concurrently. These mutants lacked the BenM and CatM transcriptional regulators that normally activate genes for benzoate catabolism. A model is presented in which BenM and CatM prevent
pobA
expression indirectly during growth on benzoate. These regulators may affect
pobA
expression by lowering the PcaK-mediated uptake of 4-hydroxybenzoate. Consistent with this model, BenM and CatM bound in vitro to an operator-promoter fragment controlling the expression of several
pca
genes, including
pcaK
. These studies provide the first direct evidence of transcriptional cross-regulation between the distinct but analogous branches of the β-ketoadipate pathway.