Functional characterization of protein kinase CK2 regulatory subunits regulating Penicillium oxalicum asexual development and hydrolytic enzyme production

• The functions of two casein kinase CK2 regulatory subunits were characterized in Penicillium oxalicum.
• CK2B2, but not CK2B1, regulates conidial germination and conidiation.
• CK2B1 and CK2B2 were similarly involved in the regulation of extracellular cellulase and amylase production.
• CK2 regulates the expression of transcriptional activators for conidiation and glycoside hydrolase production.
• The ck2B mutant phenotype of reduced cellulase production was masked by creA disruption.

Casein kinase CK2 is a ubiquitous and conserved phosphate transferase that is critical for the growth and development of eukaryotic cells. In Penicillium oxalicum, one catalytic subunit (CK2A) and two regulatory subunits (CK2B1 and CK2B2) of CK2 were annotated. In this study, CK2 regulatory subunit-defective mutants Δck2B1 and Δck2B2 were constructed to investigate the biological function of CK2 in P. oxalicum. The Δck2B1 strain exhibited minimal changes in morphogenesis and conidiation, whereas the Δck2B2 strain showed delayed conidial germination and drastically reduced conidiation compared with the parent strain. The defect in conidiation in Δck2B2 could be attributed to the reduced expression of transcription factor BrlA. Both Δck2B1 and Δck2B2 showed delayed autolysis in carbon-starvation medium compared with the parent strain. Cellulase and amylase production were decreased considerably in both mutants. The transcript abundances of the main extracellular glycoside hydrolase genes cel7A-2, bgl1, and amy15A, as well as those of three related transcriptional activators (i.e., ClrB, XlnR, and AmyR), were reduced or delayed in the mutants. Epistasis analysis suggested that CK2B1 and CK2B2 might function upstream of transcription factor CreA by inhibiting its repressing activity. In summary, CK2 plays important roles in development and extracellular enzyme production in P. oxalicum, with both unique and overlapping functions performed by the two regulatory subunits.