Changes in the structure of chromatin in pea shoots upon a shift in growth temperature from 2°C to 25°C

Low temperature-resistant pea seedlings (Pisum sativum cv. Alaska), which had been kept at 2°C in darkness, during which time growth had been almost totally suppressed, were either transferred to 25°C (warmed sample) or kept at 2°C (chilled sample) for 1 day. Rapid growth occurred at 25°C. The number of nuclei in the shoot tips of the warmed sample was twice that in the chilled sample.Nuclei prepared from the warmed and chilled samples were digested by micrococcal nuclease at the same rate and appeared to have nucleosomal repeats of similar length, namely, approximately 150 base pairs, and they generated similar patterns in terms of ladders of multimers of the basic fragment.However, significant differences in terms of chromatin structure were observed when the chromatins isolated from the warmed and chilled samples were compared. The digestibility by DNase II was apparently greater in the case of chromatin prepared from the warmed sample. The analysis of the melting curves of the chromatins revealed three phases. The T_m of each phase was higher in the case of chromatin prepared from the chilled sample. Furthermore, an analysis by sucrose density gradient centrifugation revealed that the chromatin prepared from the chilled sample migrated to a region of slightly higher density in the gradient than the chromatin from the warmed sample. Taken together, these results suggest that the structure of the chromatin prepared from the chilled sample may be more compact than that of the chromatin prepared from the warmed sample.