Cis-acting regulation of NOR cistrons in Eleutherine bulbosa (Iridaceae)

Abstract

In Eleutherine bulbosa the nucleolus organizer region (NOR) is found on the heteromorphic chromosome pair I. This shows a pericentric inversion that accounts for the presence of a metacentric and an acrocentric chromosome, and a tandem NOR duplication in the metacentric homologue. The rRNA genes are located in the secondary constriction and in the nucleolar heterochromatin, as suggested by the silver staining. The nucleolar heterochromatin appears to be slightly undercondensed in order to allow its transcription. The transcriptional activity of each NOR was evaluated by the nucleolar volume or by the secondary constriction extension. This activity was observed in tapetum cells as well as in root tip cells and microsporocytes. In the latter two cell types each homologue produced a similar sized nucleolus, whereas in tapetum cells the NOR of the metacentric chromosome was always hyperactivated whilst that of the acrocentric was underactivated. The hyperactivation was more pronounced in prophase than in interphase but was constant in diploid and tetraploid nuclei as well as in mono-and binucleate cells. This consistent change in the activity level of each homologous NOR was interpreted as being due to cis-acting transcriptional regulation. The possible mechanism of cis regulation is discussed. Cis-acting regulation of NOR cistrons in Eleutherine bulbosa (Iridaceae).