Expression and occurrence of uracil-DNA glycosylase in higher plants

Uracil-DNA glycosylase (UDG) is the first enzyme in the base excision repair pathway for removal of uracil in DNA. DNA repair capacity is likely to be a critical factor in mutagenesis and thereby in the capacity to prevent genetic damage and unwanted variation. We have studied expression of UDG in 9 higher plant species. The highest expression of UDG was measured in Solanum tuberosum. A comparison of 6 Solanum tuberosum cultivars showed that the specific activity ranged from 30 pmol mg¹ protein min^-1 in the cultivar Laila to 80 pmol mg^-1 protein min^-1 in the cultivar Ostara. Measurement of UDG in Begonia X cheimantha gave no indications of enzyme activity. The possible effects of no or low UDG activity is discussed. In vitro cultures of Solanum tuberosum and Thymus vulgaris were used to examine the effect of auxin and cytokinin on the UDG activity. Axillary shoots of Solanum tuberosum were cultured on medium including 20 variations in hormone concentration. Auxin (1-naphtaleneacetic acid) increased the expression of UDG. Plants cultured on medium supplemented with 3 mg 1^-1 1-naphtaleneacetic acid showed a specific UDG activity which was approximately 3-fold higher than the activity in controls. The cytokinin benzyladenine reduced the specific UDG activity at concentrations in the range 0.25-10 mg 1^-1 . In vitro cultured Saintpaulia ionantha was used to examine UDG activity during initiation, conditioning and multiplication cycles. In general, highest expression of UDG was measured in the conditioning cycle on hormone free medium. Measurement of UDG expression during single subculture periods, clearly showed that UDG expression may vary over one culture period. Expression of UDG was in general highest three weeks after transfer to fresh medium. Of different seedling organs from 0- to 15-day-old Brassica napus L., roots and hypocotyls showed the highest UDG activities. In cotyledons a very low and nearly constant specific activity was observed. In 12-day-old seedlings the activity in roots was approximately 20 times higher than the activity in cotyledons.