Effects exerted by transcriptional regulator PcaU from Actinetobacter sp strain ADP1

Protocatechuate degradation is accomplished in a multistep inducible catabo lic pathway in Acinetobacter sp. strain ADP1. The induction is brought abou t by the transcriptional regulator PcaU in concert with the inducer protoca techuate, PcaU, a member of the new IclR family of transcriptional regulato rs, was shown to play a role in the activation of transcription at the prom oter for the structural pea genes, leaving open the participation of additi onal activators. In this work we show that there is no PcaU-independent tra nscriptional activation at the pea gene promoter. The minimal inducer conce ntration leading to an induction response is 10(-5) M protocatechuate, The extent of expression of the pea genes was observed to depend on the nature of the inducing carbon source, and this is assumed to be caused by differen t internal levels of protocatechuate in the cells. The basal level of expre ssion was shown to be comparatively high and to vary depending on the nonin ducing carbon source independent of PcaU. In addition to the activating fun ction, in vivo results suggest a repressing function for PcaU at the pea ge ne promoter in the absence of an elevated inducer concentration. Expression at the pcaU gene promoter is independent of the growth condition but is su bject to strong negative autoregulation, We propose a model in which PcaU e xerts a repressor function both at its own promoter and at the structural g ene promoter and in addition functions as an activator of transcription at the structural gene promoter at elevated inducer concentration.