A Nicotiana tabacum cell culture selected for accelerated growth on mannose has increased expression of phosphomannose isomerase

Phosphomannose isomerase (PMI), a key enzyme in mannose (Man) metabolism, is expressed at very low levels in many plant species. For example, measured PMI activity in Nicotiana tabacum (NT1) suspension cells is relatively low, resulting in slow metabolism of Man. Not surprisingly then, NT1 cultures were observed to grow six times faster on glucose (Glc) than on Man as sole carbon source. We report here the selection of a mutant NT1 cell line that grows four times faster on Man than the parental culture from which it was derived. This cell line had fivefold greater PMI activity than the parental culture, which likely contributes to the increased growth rate on Man. The selected line continued to express elevated PMI activity after transfer to Glc, suggesting a stable genetic change. However, the selected line grew more slowly than the wild type on Glc. This was likely due to a more than 50%,, reduction in hexokinase (HK) activity, an enzyme that is required for the phosphorylation and subsequent metabolism of Glc. Unlike HK. fructokinase activity was essentially unchanged in the mutant cell lines. However, activities of the carbohydrate metabolic enzymes phosphoglucose isomerase and 6-phosphogluconate dehydrogenase were also higher in mutant cells. (C) 2003 Elsevier Ireland Ltd. All rights reserved.