Role of connexin 26 (GJB2) & mitochondrial small ribosomal RNA (mt 12S rRNA) genes in sporadic & aminoglycoside-induced non syndromic hearing impairment.

Non syndromic hearing impairment is a common sensory disorder, which affects one in 600 newborns. Though more than 50 nuclear genes are involved in causing non syndromic hearing impairment, mutations in the connexin 26 (GJB2) gene explain a high proportion of congenital deafness in several populations worldwide. The diversity of genes and genetic loci implicated in hearing loss defines the complexity of the genetic basis of hearing. This review focuses on the role of connexin 26 and mitochondrial 12S rRNA genes in hearing which will be helpful for better understanding of genes in sporadic and aminoglycosideinduced non syndromic hearing impairment.

Interaction of yeast elongation factor 3 with polynucleotides, ribosomal RNA and ribosomal subunits.

In addition to the two usual eukaryotic elongation factors (EF-1 alpha and EF-2) fungal ribosomes need a third protein, elongation factor 3, for translation. EF-3 is essential for in vivo and in vitro protein synthesis. Functionally, EF-3 stimulates EF-1 alpha dependent binding of aminoacyl-tRNA to the ribosomal A site when E site is occupied by deacylated tRNA. EF-3 has intrinsic ATPase activity which is regulated by the functional state of the ribosome. EF-3 ATPase is activated by both 40S and 60S ribosomal subunits. However intact 80S ribosomes are needed for efficient activation of EF-3 ATPase. EF-3 appears to be an RNA binding protein with high affinity for polynucleotides containing guanosine rich sequences. To determine whether guanosine rich sequence of ribosomal RNA is involved in EF-3 binding, an antisense oligonucleotide dC6 was used to block EF-3 interaction with the ribosome. The oligonucleotide suppresses activation of EF-3 ATPase by 40S ribosomal subunit and not by the 60S or the 80S particles. Poly(U)-directed polyphenylalanine synthesis by yeast ribosomes is inhibited by dC6. To define the binding site of the oligonucleotide and presumably of EF-3 on 18S ribosomal RNA, hydrolysis of rRNA by RNase H was followed in the presence of dC6. These experiments reveal an RNase H cleavage site at 1094GGGGGG1099 sequence of 18S ribosomal RNA. This guanosine rich sequence of rRNA is suggested to be involved in EF-3 binding to yeast ribosome. Data presented in this communication suggest that the activity of EF-3 involved a direct interaction with the guanosine rich sequence of rRNA.

Hormonal regulation of ribosomal RNA synthesis in immature rat uterus

Fraçöes nucleares e nucleolares de útero de ratas jovens injetadas com 17-ß-estadiol näo radioativo foram isoladas e caracterizadas morfológica e bioquimicamente. A análise destas preparaçöes revelou que estas encontravam-se substancialmente puras, com a alfa-amanitina inibindo apenas apenas 20%% da atividade transcricional em nucléolos isolados. O emprego destas fraçöes nos experimentos utilizando a técnica de troca do 3H-estradiol pelo 17-ß-estradiol näo radioativo injetado préviamente nos animais, levou à determinaçäo de curvas de cinética, saturaçäo e de Scatchard para núcleos e nucléolos. Determinou-se alguns parâmetros físico-químicos que definem a ligaçäo do complexo receptor 17-ß-estradiol em núcleos e nucléolos. Os resultados obtidos mostram que tanto núcleos quanto nucléolos apresentam sítios aceptores para o complexo receptor-17-ß-estradiol, sendo esta ligaçäo saturável e específica devido à inibiçäo por um inibidor competitivo,m o diestilstilbestrol, com valores de Kd = 1,06 x 10**-9 M e 20,41 x10**-8 M; Ka = 0,94 x 10**10 M e 20,05 x 10 M; Bo = 0,49 x 10**9 M e 0,35 x 10**-9 M para nucléolos, respectivamente. Em nucléolos a curva de cinética sugere uma acumulaçäo tardia do complexo receptor-hormônio de características diferentes daquelas apresentadas em núcleos, onde existe uma acumulaçäo precoce deste complexo. A partir dos resultados apresentados neste estudo, conclue-se que em útero de ratas jovens pode haver uma açäo direta do hormônio sobre os genes ribossomais. Tal mecanismo caracteriza um sistema de regulaçäo extremamente preciso e complexo, com a presença de sítios aceptores específicos para a açäo do complexo receptor-hormônio sobre o genoma ribossomal. Tendo estabelecido este ponto, estudos adicionais säo necessários para determinar os mecanismos pelos quais os receptores estimulam as velocidades de transcriçäo e suas possíveis implicaçöes no controle gênico