High impact information on B4

• Addition of cell lysates containing baculovirus-expressed cyclin to these extracts induces the polyadenylation of exogenous B4 RNA [1].
• To identify factors of the polyadenylation apparatus that might be responsible for the activation, we employed UV cross-linking and identified a 58-kD protein that binds the B4 CPE in oocyte extracts [1].
• Northern analysis showed that Xenopus B4 mRNA is expressed in oogenesis and embryogenesis through to the neurula stage [2].
• Immunocytochemistry with anti-B4 antibodies showed that B4 protein is only detectable in preneurula stages; it is localized to nuclei and is associated with metaphase chromosomes [2].
• The B4 protein also has certain other H1 protein characteristics--a tripartite structure consisting of a mainly hydrophobic central domain flanked by an amino-terminal segment and a long hydrophilic carboxyterminal tail containing a tandemly repeated amino acid motif [2].

Biological context of B4

• However, in contrast to histone H1 mRNA, B4 mRNA has a classic polyadenylation signal, is polyadenylated, and lacks the histone H1 3' noncoding consensus sequence involved in RNA processing [2].
• However neither B4 nor core histone phosphorylation are required for the assembly of spaced nucleosomal arrays [3].
• We examined the structural and functional consequences of incorporating either histone H1, histone B4 or HMG1 into a synthetic dinucleosome containing two 5S rRNA genes [4].
• For histone B4 mRNA, the Mos response elements were in the coding region or 5' UTR [5].
• In contrast, the CS H1 protein displays highest sequence homology with the H1M (B4) histone of Xenopus laevis [6].

Anatomical context of B4

• Incubation of fertilized eggs with cycloheximide prevented the embryonic inhibition of cdk2 RNA polyadenylation but did not affect the robust polyadenylation of B4 RNA [7].

Physical interactions of B4

• Results presented in three recent articles 1-3 together demonstrate that replacement of the cleavage stage linker histone B4 by somatic H1 leads to chromatosomes positioned directly over these genes and adjacent sequences so as to occlude the binding site for the critical transcription factor TFIIIA [8].

Other interactions of B4

• However, polyadenylation of cyclin B1 and histone B4 mRNA was still observed [5].

Analytical, diagnostic and therapeutic context of B4

• Immunoblotting revealed approximately constant levels of B4 protein per embryo for the first 2 days of development [2].
• Gel mobility shift assays show that two polyadenylation complexes are formed on B4 RNA [9].

References