ATPDependent RNA Helicase

We cloned and characterized a salt-responsive transcript, HVD1 (Hordeum vulgare DEAD box protein) (Fig. 21.2), encoding a putative ATP-dependent RNA helicase belonging to a group known as the DEAD box family.14 The DEAD box family was defined first by Linder et al14 and shares several highly conserved motifs among the different organisms ranging from E. coli to human . Most of the members of the DEAD box family have determined or putative ATP-dependent RNA helicase

Bnucl

Barley

Zinnia

GKEGHYMTMKIADGFL GKEGHYMTâ„¢KIADGFL

keghr

Fig. 21.1. Alignment of N-terminal regions of the deduced amino acid sequence of Bnuc1 with barley nuclease3 and Zinnia nuclease.13 X represents ambiguous residues. Black boxes show identical amino acids. The unknown sequence is represented by dashes.

activity, modulating RNA secondary and tertiary structure. They are involved in diverse biological functions such as translation initiation, RNA splicing, ribosome assembly and spermatogenesis. In plants, the DEAD box genes encoding eukaryotic initiation factor 4A (eIF-4A) were cloned from tobacco,15 wheat16 and rice.17 The other type of DEAD box gene was cloned from tobacco;18 it ts similar to human p68 of a DEAD box RNA helicase family, but its function is unknown.

HVD1 mRNA was detected at low levels in leaves and roots of non-stressed plants, whereas its accumulation was induced 8-fold higher under salt stress. HVD1 cDNA encoded a 764 amino acid polypeptide with molecular mass 81.8 kDa. The deduced HVD1 protein has the eight consensus motifs of the DEAD box family. In addition, the protein contained five repeats of "RGG" known as an RNA-binding motif at its hydrophilic C-terminus, showing a novel DEAD box protein induced by salt stress in plants. The level of mRNA was increased not only by salt but also by cold treatment. It is anticipated that HVD1 protein regulates expression of genes concerned with salt tolerance or important metabolisms such as photosynthesis.

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