The chemical composition, lectin content and nutritional value of selected cultivars of kidney beans (Phaseolus vulgaris) were determined. Mainly due to different amounts of reserve globulins deposited, seed N content varied between 3.4 and 5.0%. The amino acid composition and the corresponding chemical score values of 40-60 however, were similar for all the beans, with the sulphur-containing amino acids limiting. Despite similarities in composition, 11 out of the 13 cultivars examined were highly toxic for rats in the raw state. These were all found to contain high concentrations of haemagglutinating lectins (over 10% of the total protein). On the other hand, two low-lectin bean varieties, Pinto 111 and Great Northern, had no appreciable disruptive effects on the intestines and were essentially non-toxic for rats.

To better understand the mechanisms that regulate stable RNA synthesis, we have analyzed the RNA polymerase I and III transcriptional activities of extracts isolated from cells propagated under a variety of conditions. Under balanced growth conditions the levels of both RNA polymerase I-and III-specific transcription increased proportionally with growth rate. Upon nutritional starvation, RNA polymerase I transcription rapidly declined, followed by 5 S rDNA and eventually tDNA transcription. Transcriptional activities in extracts were restored when the nongrowing cultures were resuspended in fresh medium, although growth did not resume. The differential expression of 5 S rDNA and tDNA genes in extracts prepared from cells subjected to partial starvation was traced to a 5 S rDNA-specific inhibitor and not to a defect in any RNA polymerase III transcription factor. Characterization of this inhibitor indicated that it was not 5 S rRNA. It was sensitive to phenol extraction and resistant to RNase, and its target did not appear to be transcription factor IIIA. Not all treatments that slowed or stopped growth down-regulated the stable RNA transcription apparatus. Cells that have been subjected to either energy starvation or cycloheximide treatment still retain the ability to synthesize stable RNA in vitro, suggesting the presence of alternative regulatory mechanisms.

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Inclusion of pure lectins isolated from the seeds of kidney bean (Phuseolus vulgaris L. cv. h-ocessor) in diets for rats increased both faecal and urinary losses of N and resulted in a negative N balance for the animals. The relationship between total body N change [ y (mg)] and lectin concentration [x (g kg-I)] of the diet was: y = 421.5 -75.1 x, which was statistically significant ( P = 0.05). These rats developed circulating antibodies of low avidity to the dietary lectins, while no other proteins of the diet elicited a similar antibody response. Additionally, in several serum samples from rats which had been fed raw beans, the presence of small amounts of a protein reactive with rabbit anti-lectin antibodies was detected. This protein was isolated from the immune precipitate and was shown by sodium dodecylsulphate-gel electrophoresis to contain a protein subunit of 30000, which was very similar to that of pure lectins. It is tentatively suggested that lectin toxicity results from the combined effects of the interference with normal intestinal digestion and/or absorption of N through the damaged enterocytes and of systemic immune (and other) responses of the rat to the internalised lectin.

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