Konjac glucomannan (KGM) has been shown to increase human colon microbial ecology and reduce faecal toxicity in mice. The main goal of the present study was to assess the effects of a KGM supplement into a low-fibre diet on precancerous markers of colon cancer in a double-blind, placebo- and diet-controlled study. Adult volunteers consumed defined diets supplemented with konjac (4·5 g/d) or placebo (maize starch) for 4 weeks. Stools collected before and at the end of the supplementation were analysed for β-glucosidase, β-galactosidase and β-glucuronidase activities, microflora and bile acids. Faecal water was co-incubated with Caco-2 cells, a model of human colonocytes, to determine the cytotoxicity and DNA-damaging effect as assessed by the comet assay. The results indicated that the KGM supplement significantly decreased faecal β-glucuronidase activity by 25·6 (se 7·8) % and faecal secondary bile acid level by 42·4 (se 11·8) %. In contrast, consuming the defined diet supplemented with placebo for 4 weeks did not improve these determinants. The KGM-supplemented diet, but not the placebo diet, significantly increased the survival rate (%) of Caco-2 cells co-incubated with faecal water for 1 and 3 h, respectively. In addition, KGM significantly reduced the DNA damage induced by the faecal water alone or in combination with H2O2. The faecal bifidobacteria and lactobacilli levels increased only with the KGM-supplemented diet. Therefore, we conclude that supplementation of KGM into a low-fibre diet improved the faecal microbial ecology and metabolites, which may contribute to the reduced toxicity of faecal water and precancerous risk factors of human colon cancer.

Target regions for posttranscriptional silencing of transgenes often reside in the 3′ region of the coding sequence, although there are exceptions. To resolve if the target region is determined by the gene undergoing silencing rather than by the structure of the transgene loci or the plant genetic background, we have performed detailed analyses of target regions in three spontaneously β-glucuronidase (GUS) silencing tobacco lines of different origin. From quantitative cosuppression experiments, we show that the main target region in all three tobacco lines is found within the 3′ half of the GUS coding region but upstream of the last 200 nt. The quantities of small (21–25 nt) RNAs homologous to 5′ or 3′ regions of the GUS coding sequence were found to correlate approximately with the target strength of the corresponding regions. These results suggest that transgene locus structure and plant genetic background are not major determinants of silencing target regions. We also show that virus-induced gene silencing (VIGS) of GUS in Nicotiana benthamiana is induced equally effectively with Potato virus X carrying either the 5′ or 3′ third of the GUS coding region. This indicates that both regions can act as efficient inducers as well as targets of posttranscriptional silencing, although the 3′ region is the predominant target region in the spontaneously silencing transgenic plant lines examined. Finally, we investigated spreading of the target region in the N. benthamiana plants undergoing VIGS. Surprisingly, only evidence for spreading of the target region in the 5′-3′ direction was obtained. This finding may help explain why the majority of target regions examined to date lie within the 3′ region of transgenes.