Effect of N-methyl-N-nitrosourea on avian erythrocyte nuclei reactivation

Avian erythrocytes are terminally differentiated cells but they can be reactivated by fusion with actively metabolising cells. We have examined the effects of treating the erythrocytes with a carcinogenic methylating agent, N-methyl-N-Nitrosourea (MNU), on the process of reactivation of adult and embryonic nuclei. We have found that the rate of nuclear enlargement is slightly lower in nuclei from MNU-treated cells than from control cells and that there is a marked delay of about 24 h in the appearance of nucleoli in both adult and embryonic cells. This is not due to an effect of MNU on ribosomal (r)DNA: the number of rDNA genes appears to be similar in treated and control cells. Also, the number of rDNA genes appears to be similar in adult and embryonic cells and in unreactivated and reactivated embryonic nuclei: thus, differences in reactivation rate between adult and embryonic cells, observed by us and others, can not be attributed to a gross difference in their ribosomal DNA contents, and reappearance of nucleoli on reactivation can not be due to an amplification of rDNA (i.e., to recovery of such genes if lost on terminal differentiation). We suggest that MNU, although a monofunctional alkylating agent, may cause increased association--possibly cross-linkage--between DNA and protein in chromatin, thereby hindering access of host cell reactivating proteins, especially to the nucleolar regions.