27. Regulatory elements for expression of ribosomal RNA genes resides in the
(1) transcribed spacer region
(2) non-transcribe spacer region
(3) 5’ flanking region of individual ribosomal genes
(4) internal regions within the genes

Regulatory Elements for Expression of Ribosomal RNA Genes in Eukaryotes

Correct Answer:

(2) non-transcribed spacer region

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Detailed Explanation:

Ribosomal RNA (rRNA) genes in eukaryotes are organized in tandem repeats consisting of the rRNA coding sequences separated by intergenic spacer regions (IGRs), also called non-transcribed spacer (NTS) regions. These NTS regions lie between the rRNA gene repeats and contain most of the regulatory elements necessary for rRNA gene transcription.

• The non-transcribed spacer region includes important regulatory sequences such as promoters, enhancers, and terminators that control the transcription of the rRNA genes by RNA polymerase I.

• The transcribed spacer regions (internal and external transcribed spacers) are part of the precursor rRNA transcript and are removed during rRNA processing; they do not contain regulatory elements for transcription initiation.

• The 5’ flanking region of individual ribosomal genes may contain promoter elements, but in many eukaryotes, the key regulatory elements are located within the larger non-transcribed spacer between gene repeats.

• Some regulatory elements can also be found internally within rRNA genes (e.g., internal promoters for 5S rRNA transcribed by RNA polymerase III), but for the main rRNA genes (18S, 5.8S, 28S), the non-transcribed spacer is the primary regulatory region.

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Supporting Evidence from Literature:

• The intergenic spacer (IGS), or non-transcribed spacer, contains enhancers, spacer promoters, and other regulatory sequences that modulate rRNA transcription efficiency and chromatin structure (ScienceDirect).

• The rDNA repeats are arranged with a transcribed region and an intergenic spacer; the latter harbors most transcription regulatory elements (PMC).

• Studies in various eukaryotes, including Xenopus and yeast, show that the non-transcribed spacer is critical for controlling rRNA gene expression.

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Summary Table

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Conclusion

The non-transcribed spacer region (also known as the intergenic spacer) is the primary location of regulatory elements controlling the expression of ribosomal RNA genes in eukaryotes. It contains promoters, enhancers, and terminators essential for RNA polymerase I transcription regulation.