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The activity of telomerase in plant cells is precisely regulated in response to changes in cell division rate. To explore this regulatory mechanism, the effect on telomerase activity of protein extracts from nuclei of telomerase‐negative tissues was examined. An inhibition of telomerase activity was found which was species‐non‐specific. This inhibition was due to proteins which form salt‐stable, sequence‐specific complexes with the G‐rich telomeric strand and reduce its accessibility, as shown by gel retardation and by terminal transferase (TdT) extension of G‐rich telomeric and non‐telomeric (substrate) primers. A 40 kDa polypeptide was detected by SDS–PAGE after cross‐linking the complex formed by extracts from tobacco leaf nuclei. Such proteins may be involved in regulation of telomerase activity in plants.

The activity of telomerase in plant cells is precisely regulated in response to changes in cell division rate. To explore this regulatory mechanism, the effect on telomerase activity of protein extracts from nuclei of telomerase-negative tissues was examined. An inhibition of telomerase activity was found which was species-non-specific. This inhibition was due to proteins which form salt-stable, sequence-specific complexes with the G-rich telomeric strand and reduce its accessibility, as shown by gel retardation and by terminal transferase (TdT) extension of G-rich telomeric and non-telomeric (substrate) primers. A 40 kDa polypeptide was detected by SDS–PAGE after cross-linking the complex formed by extracts from tobacco leaf nuclei. Such proteins may be involved in regulation of telomerase activity in plants.

Changes in chromatin structure of the HRS60 family of repetitive sequences in tobacco DNA were studied after hypomethylation induced with 5‐azacytidine or DL‐ethionine. The TaqI site in the HRS60 units lies in nucleosomal core regions and its cleavage is enhanced in the hypomethylated chromatin. In contrast, the cleavage of the Sau3A1 site located in linker DNA does not depend on the level of methylation of DNA.

Changes in chromatin structure of the HRS60 family of repetitive sequences in tobacco DNA were studied after hypomethylation induced with 5-azacytidine or DL-ethionine. The TaqI site in the HRS60 units lies in nucleosomal core regions and its cleavage is enhanced in the hypomethylated chromatin. In contrast, the cleavage of the Sau3A1 site located in linker DNA does not depend on the level of methylation of DNA.

Programmed cell death (PCD) may be triggered by a variety of environmental stimuli. In this report we show that low temperature treatment of tobacco BY-2 cells results in specific chromatin changes. The early stage was characterised by chromatin condensation associated with specific endonucleolytic cleavage of the genome into fragments of 50–100 kbp in size. Later, after 2 weeks of the cold treatment, a ladder of nucleosomal units (178 bp) and their multiples occurred. Chromatin changes were accompanied by a general decrease in cell viability. However, the cell culture retained about 11% of living cells even after prolonged incubation in the cold suggesting the presence of a cold-resistant population of cells. The results support the view that PCD was activated by the cold stress. We suggest that cold-stressed tobacco BY-2 culture might be a useful system for investigation of PCD in plant cells.

Telomerase is a ribonucleoprotein enzyme which elongates the G‐rich strand of telomeric DNA to compensate for the progressive reduction in its length due to incomplete replication of chromosome ends, which in human somatic cells leads to cell cycle arrest upon shortening of telomeres to a critical length. To examine the possible involvement of telomerase in metabolism of plant genetic material, we used cells of Nicotiana tabacumstrain TBY‐2, a stable long‐term culture which has kept a constant pattern of restriction fragments from chromosome termini during its 6 month period of cultivation in our laboratory. In a direct assay for telomerase, a 5′ end‐labeled plant telomeric oligonucleotide 5′(TTTAGGG)63′was elongated in a TBY‐2 cell extract, showing a pausing pattern which is a characteristic feature of telomerases from other organisms. The elongation was inhibited by RNase A pretreatment of the extract. We conclude that plant cells possess telomerase which is used for maintenance of their telomeres.

PCR and primers derived from the telomeric repeat (CCCTAAA)nand from the tobacco subtelomeric tandemly repetitive sequence HRS60 (EMBL X12489) were used to amplify the region linking the two loci. A 131 bp PCR product was obtained both from total tobacco DNA and from the DNA fraction enriched for telomeres. Its sequence only consists of the telomeric primer and the attached region of the HRS60 repetitive unit up to the end of the sequence complementary to the HRS60 primer. The site of direct continuity between the two sequences is formed by a (dA)7tract.