SummaryMaize embryonic axes contain stored mRNAs, some of which are able to undergo cap-independent translation initiation during germination. The Hsp101 mRNA, encoding a heat shock protein, is essential for thermotolerance induction and is present among the stored transcripts. This research aimed to investigate whether the Hsp101 transcript is IRES-driven regulated upon heat stress. Hsp101 transcribed either in vitro or in vivo was efficiently translated via a cap-independent mechanism. This was observed either in an animal in vitro translation system containing proteolytically cleaved eukaryotic initiation factor eIF4G or in a plant system lacking both eIF4E and eIFiso4E initiation factors. Deletion of the 5¢ untranslated region (UTR) from the Hsp101 mRNA abolished its cap-independent translation indicating that this nucleotide sequence is required to confer cap-independent initiation. Bicistronic constructs containing the Hsp101 mRNA 5¢UTR in sense and anti-sense directions between two reporter genes were translated in both cap-independent systems. A similar bicistronic construct containing a viral internal ribosome entry site (IRES) element between the reporter genes was used as control. Internal translation of the second reporter gene was observed when the Hsp101 5¢UTR was in the sense but not in the anti-sense orientation in the bicistronic construct. Taken together, these data suggest that the 5¢UTR of maize Hsp101, a plant cellular mRNA, functions as an IRES-like element accounting for its capindependent translation during heat stress.

In order to investigate a putative neurotransmitter function of γ-aminobutyric acid (GABA) in the rat vestibule we chose to study the cellular localization and properties of glutamic acid decarboxylase (GAD), the rate-limiting enzyme for the synthesis of GABA, in the rat ampullary cristae, with respective immunocytochemical and immunoblotting techniques, using an extensively characterized rabbit anti-GAD serum. With these procedures we found a GAD-like immunopositivity in the sensory cells and in fibers of the crista ampullaris stroma. In ampullary cristae homogenates, a GAD composed of at least two subunits (53 and 67 kDa which were present also in rat brain and cerebellum homogenates) was encountered. These findings suggest that GAD present in the rat vestibule is homologous to the brain enzyme and it has the appropriate localization to synthesize GABA to be used as a neurotransmitter in the rat vestibule.