The centromeric regions in the fission yeast Schizosaccharomyces pombe are wrapped by nucleosomes containing the CENP-A histone H3 variant, and are flanked by clusters of tRNA genes that separate them from the surrounding pericentric heterochromatin. If the tRNA clusters are removed from this region, the HP1-bound heterochromatin spreads further to cover the centromeric regions. The tRNA genes are transcribed by strong RNA polymerase III promoters, which can associate with transcription factors and recruit chromatin-modifying enzymes.
Based on these observations, indicate which blanks (A to E) in the paragraph below correspond to each of the following phrases. Your answer would be a five-letter string composed of letters A to E only, e.g. BCDEA.
"The …(A) are not sufficient to prevent heterochromatin expansion to the centromeric regions. Instead, the …(B) are acting as …(C) in S. pombe, similar to the role of the …(D) in the ?-globin locus in chickens and humans. Likely candidates for the histone-modifying enzymes recruited by the RNA polymerase III complexes are …(E)."
( ) HS4 element
( ) chromatin boundaries
( ) histone acetyl transferases
( ) tRNA genes
( ) CENP-A-containing histones

Question

The centromeric regions in the fission yeast Schizosaccharomyces pombe are wrapped by nucleosomes containing the CENP-A histone H3 variant, and are flanked by clusters of tRNA genes that separate them from the surrounding pericentric heterochromatin. If the tRNA clusters are removed from this region, the HP1-bound heterochromatin spreads further to cover the centromeric regions. The tRNA genes are transcribed by strong RNA polymerase III promoters, which can associate with transcription factors and recruit chromatin-modifying enzymes.
Based on these observations, indicate which blanks (A to E) in the paragraph below correspond to each of the following phrases. Your answer would be a five-letter string composed of letters A to E only, e.g. BCDEA.
"The …(A) are not sufficient to prevent heterochromatin expansion to the centromeric regions. Instead, the …(B) are acting as …(C) in S. pombe, similar to the role of the …(D) in the ?-globin locus in chickens and humans. Likely candidates for the histone-modifying enzymes recruited by the RNA polymerase III complexes are …(E)."
( )	HS4 element
( )	chromatin boundaries
( )	histone acetyl transferases
( )	tRNA genes
( )	CENP-A-containing histones

Quizplus · Accepted Answer

In this example, the tRNA genes serve as barrier sequences to block the spread of heterochromatin by recruiting histone-modifying enzymes such as histone acetyl transferase enzymes. This is similar to the role of the HS4 sequence in protecting the ?-globin locus in our red blood cells. Heterochromatin can spread over the centromeric regions in the absence of the tRNA genes in the fission yeast, suggesting that the presence of CENP-A-containing histones does not provide a sufficient protection.

The Centromeric Regions in the Fission Yeast Schizosaccharomyces Pombe Are