Transcript analysis of penicillin genes from Penicillium chrysogenum

The presence of a transcriptional control simultaneously affecting the expression of the three penicillin biosynthetic genes, pcbAB, pcbC, and penDE (pen genes), was demonstrated in Penicillium chrysogenum. Using probes specific to each gene, it was observed that the highest level of expression of the pen genes occurred during exponential growth, in both the original ancestral strain (NRRL1951) and a high-penicillin producing strain P2. Expression also occurred in the presence of high concentrations of glucose, indicating that carbon catabolite repression was not directly involved in the regulation. Transcription of the pen genes appeared to cease as the growth rate decreased. Growth was limited in a fermenter by the rate of oxygen transfer. The phosphoglycerate kinase gene (pgk), used as a control, was strongly induced by the reduced oxygen levels, suggesting a stress-related response. By maintaining optimum growth conditions in fermenters, no induction of the pgk gene was observed whereas expression of the pen genes could be maintained. It was also possible to re-establish expression of the pen genes, after normal cessation, by the addition of cycloheximide to the culture medium.