Effects of restriction endonucleases on nucleolar organizing regions in the ant Tapinoma nigerrimum

The effects of some restriction endonucleases (REs) on the nucleolar organi zing regions and on the genes for ribosomal RNA (rDNA) were analyzed using the nucleolar organizing region of the chromosome 6 of Tapinoma nigerrimum as an experimental model, since, in accordance with previous studies, the g enes for ribosomal RNA seem to be present only in this chromosome. In situ non-digestion of the nucleolar organizing region was observed when EcoRI an d HindIII were used. However, very evident digestion and partial digestion respectively were observed when HaeIII and Tru9I were used. Southern blot a nalysis realized on naked DNA digested with the same REs and using rDNA of Drosophila melanogaster as probe showed that there are target sequences for these enzymes in the rDNA. In accordance with the results obtained, the rD NA is poor in EcoRI and HindIII sequences, contains moderate amounts of Tru 9I sequences, and is rich in HaeIII sequences. All the data obtained sugges t that in the nucleolar organizing region of Tapinoma nigerrimum, the major , if not the only, limiting factor affecting in situ digestion by the REs u sed is the presence and frequency of their specific restriction targets. Co nsequently, extraction of DNA from this chromosome region depends on the si ze of the fragments originated.