Laccase-less mutants of the white-rot fungus Pycnoporus cinnabarinus cannot delignify kraft pulp

The white-rot fungus Pycnoporus cinnabarinus produces only one isoform of laccase and traces of a peroxidase which is neither manganese peroxidase (MnP) nor lignin peroxidase (LiP). To test the impact of laccase on pulp bleaching we performed studies with P. cinnabarinus wild type (WT) and its laccase-less mutants on softwood kraft pulp (SWKP) in both static and shake flask cultures. In the static pulp cultures (at 20% pulp consistency in water), after 25 days incubation, P. cinnabarinus wild type increased the pulp brightness from 35 to 59% ISO, and decreased the kappa number from 15 to 5.7. However P. cinnabarinus laccase-less mutants 51 and 85 increased the brightness to only 40.6 and 40.7, and decreased the kappa number to 12 and 13.3, respectively. In the shake flask cultures with pulp suspended in water (at 2% consistency), the wild type increased pulp brightness from 38 to 52.4% ISO, and decreased the kappa number from 12 to 5.9. The laccase-less mutants 51, and 85 increased the brightness only to 43.8 and 40, and decrease the kappa number to 9.2 and 10.6, respectively. These results showed that both in the static and shaking cultures, the laccase-less mutants were very inefficient or unable to bleach pulp which is a clear indication of the importance of laccase for lignin degradation and pulp bleaching by P. cinnabarinus. (C) 1998 Elsevier Science B.V. All rights reserved.