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Cell Biology International (2003) 27, 247248 (Printed in Great Britain)
Position-dependent gene activity in cytologically reconstructed barley karyotypes
N. Papazova* and K. Gecheff
AgroBioInstitute, 2232-Kostinbrod-2, Bulgaria
*Corresponding author. Tel.: +359-721-2552; fax: +359-721-4985.
Structural chromosomal mutations that appear spontaneously or are experimentally induced are of great importance for elucidating some theoretical problems in genetics, and also for breeding practice. One of the most significant consequences of the structural rearrangements is the change of the gene position, which in many cases can alter the gene expression. This so-called position effect in plants is not well studied. Knowledge of the mechanisms underlying the position-dependent gene activity is of great interest for manipulating gene expression by alteration in the gene position. Our studies were focused on translocation-induced intraspecific nucleolar dominance in barley which is a very suitable model system for studying position-dependent gene activity.
2 Results and discussion
2.1 Influence of the chromosome position of the ribosomal genes on their expression in translocation lines
A number of cytologically reconstructed karyotypes of barley (Hordeum vulgare L. cv. Freya) (Gecheff, 1989) containing chromosomal rearrangements which modified the position and structure of ribosomal RNA genes have been used as a model system for studying the position-dependent gene activity.
The detection of the activity of ribosomal genes was based on the positive silver reaction of nucleolus organizing regions (NORs) of metaphase chromosomes and of the number and size of silver stained nucleoli in somatic interphase cells and meiocytes. The positive reaction of NORs with silver nitrate is due to affinity of the nucleolus proteins: nucleolin, B
The activity of rRNA genes was also analysed in meiocytes. The data support the model of formation and fusion of the primary nucleoli observed in somatic cells (Nicoloff et al., 1977). The most suitable stage for this analysis proved to be pachytene.
The role of cytosine methylation, which is of great importance for the control of specific rRNA gene repression in interspecific hybrids (Flavell et al., 1988; Jupe and Zimmer, 1990; Neves et al., 1997), has been investigated. Our results showed the limited role of this epigenetic mechanism in the regulation of specific rRNA gene silencing in barley translocation lines (Papazova et al., 2001).
2.2 Transcriptional activity of inversion split NOR
Interesting results have been obtained after analysis of karyotype PK-88-4 consisting of a pericentric inversion splitting the NOR7 into two unequal parts residing on the opposite arms of chromosome 7. The presence of two silver bands at chromosome 75i, of more than four nucleoli in interphase nuclei as well as three pachytene nucleoli provides evidence that both parts of the split NOR are transcriptionally active (Georgiev et al., 2001).
Our data provide evidence that the intraspecific nucleolar dominance results from interchromosomal interactions, probably of cis-acting regulatory factors. While the combination of NORs 6 and 7 on opposite arms of chromosome 6 (6H) resulted in specific supression of NOR7 (Nicoloff et al., 1977; Rieger et al., 1979; Schubert and Künzel, 1990), rDNA repeats originating from the same gene family showed proportional activity when they are rearranged by pericentric inversion on opposite arms of a single chromosome (chromosome 7 of karyotype PK-88-4). Considering additionally the equal activity of both NORs 6 on isochromosome 6s (Schubert and Künzel, 1990), translocation-induced intraspecific nucleolar dominance in barley seems to be due to the interaction of NOR6 and NOR7 themselves or to interaction of other genetic factors external to these loci.
Nikoloff H, Anastassova-Kristeva, M, Künzel, G, Rieger, R. The behavior of nucleolus organizers in structurally changed karyotypes of barley. Chromosoma 1977:62:103-9
Schubert I, Künzel, G. Position-dependent NOR activity in barley. Chromosoma 1990:99:352-9