Translational attenuation has been proposed to be the mechanism by which the erm(B) gene is induced. Here, we report genetic and biochemical evidence, obtained by using erythromycin as the inducing antibiotic, that supports this hypothesis. We also show that erythromycin increases the level of the erm(B) transcript by stalling the ribosome on the leader mRNA and thereby facilitating the stabilization and processing of the mRNA. Erythromycin-induced mRNA stabilization and processing were observed with an ochre stop at codons 11 to 13 of the leader but not with an ochre stop at codon 10. This suggests that erythromycin does not stall the ribosome before codon 11 of the leader reaches the aminoacyl site. Secondary structure analyses of the erm(B) transcripts by in vitro and in vivo chemical probing techniques identified conformational changes in the transcripts that result from induction by erythromycin. These findings demonstrate that stalling of erythromycin-bound ribosomes at leader codon 11 causes the refolding of mRNA into a conformation in which the translational initiation site for the structural gene is unmasked and renders erm(B) translationally active.Dimethylation of a single adenine in 23S rRNA causes highlevel cross-resistance to macrolide-lincosamide-streptogramin B (MLS B ) antibiotics. The erm gene encodes the methylase responsible for this modification and remains the most widespread determinant of MLS B resistance. The expression of erm can be either constitutive or inducible. When the expression is inducible, there is variation in induction specificity. All of the MLS B antibiotics act as inducers of erm(B) to various degrees (18). This feature of erm(B) is different from the features of other classes of erm, the expression of which is induced by only certain specific MLS B antibiotics (1,17,30).Translational attenuation is believed to control the expression of erm(A), erm(C), and erm(D) (14,23,31). For erm(C), the translational attenuation mechanism has been well elucidated (31). The structural gene of erm(C) is preceded by a leader peptide that comprises 19 amino acids encoded by a regulatory sequence. Without an inducer, the erm(C) mRNA is synthesized, but in a translationally unfavorable conformation. With the inducer, the inducer-bound ribosome stalls on the leader sequence with leader peptide codon 9 occupied by its aminoacyl site (A site), which leads to the conformational isomerization of the mRNA to the translationally active form. The ribosome stalling concomitantly brings about the stabilization of erm(C) mRNA owing to the blockade of its 5Ј end from the access of a nuclease (3).Despite its clinical importance, the induction mechanism of the erm(B) gene has not been elucidated in detail (18). A translational attenuation model has been proposed for erm (B) on the basis of sequence analysis (12), and enhancement of the transcript corresponding to erm(B) mRNA has also been reported (27). The erm(B) transcript contains a 5Ј leader sequence of 259 nucleotides (nt) which has the potential...

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ABSTRACT:Thermal decomposition kinetics of diglycidyl ether of bisphenol A (DGEBA)/4,4Ј-methylene dianiline (MDA) system with rubber-modified MDA was studied by the methods of Ozawa, Kissinger, and Friedman, and the kinetic parameters were compared. The thermal decomposition data of the cured epoxy resin were analyzed by thermogravimetric analysis (TGA) at different heating rates. TG curves showed that the thermal decomposition of the epoxy system occurred in one stage regardless of rubber-modified MDA content. The apparent activation energies for the DGEBA/MDA system with 10 phr of rubber-modified MDA, as determined by the Ozawa, Kissinger, and Friedman methods, are 184, 182, and 222 kJ/mol, respectively. The thermal stability of the epoxy system increased with the increasing content of rubber-modified MDA, which has four benzene rings with high thermal resistance due to the resonance structure.

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