RIBONUCLEIC-ACID TREATMENT ALTERS GENE-EXPRESSION IN DIPLOID STRAINS OF ASPERGILLUS-NIDULANS

Physical and chemical agents that promote DNA damage can induce high l evels of mitotic crossing-over in eukaryotic diploid cells. Similarly, foreign DNA segments introduced by transformation processes, in the c ell genome, can also induce mitotic crossing-over as an outcome of the reactions leading to chromosomic balance or due to the mechanisms aim ing at the integration of the exogenous DNA. Zucchi et al. have descri bed a system showing that RNA treatments are capable of inducing chang es in the genome of haploid receptor strains of Aspergillus nidulans. To verify the genetic consequences of this process in diploid cells, c onidia from two strains of this fungus were protoplastized, treated wi th homologous RNA and analyzed. Alterations in the gene expression and in the mitotic crossing-over frequencies between linked markers were detected. Among the main observed effects there was a generalized alte ration in gene expression which was very likely caused by a reversible gene inactivation mechanism due to the methylation of cytosine residu es. This was confirmed by treating the haploid segregants with the hyp omethylating agent 5-azacytidine, that restored the original gene acti vity. The presence of a duplicated segment in the chromosome I of one of the treated diploids, interfered with the RNA general effects on it s genome.