A PHOSPHATE-STARVATION INDUCIBLE BETA-GLUCOSIDASE GENE (PSR3.2) ISOLATED FROM ARABIDOPSIS-THALIANA IS A MEMBER OF A DISTINCT SUBFAMILY OF THE BGA FAMILY

We have previously isolated a phosphate starvation-response (psr) cDNA clone, psr3.1, from Brassica nigra which encodes a beta-glucosidase. Southern blots of Arabidopsis thaliana genomic DNA probed with the psr 3.1 cDNA indicated that this gene exists as a single locus. A genomic library of A. thaliana was screened at high stringency to isolate the corresponding genomic clone. The resultant clone was coined psr3.2 bec ause of its sequence divergence from isolated psr3.1 cDNA clones. Nort hern blotting with probes derived from the coding region of the genomi c clone showed that this gene is expressed at high levels in P-i-starv ed roots and the enhancement occurred within two days of growth in med ium lacking P-i. The expression of this gene is repressed by heat shoc k and anaerobic conditions, and it is not significantly induced by hig h salinity, or by nitrogen or sulfur deprivation. Sequence analysis of the genomic clone revealed the existence of 13 exons interrupted by 1 2 AT-rich introns and it possessed a high homology with the B. nigra p sr3.1 as well as various other beta-glucosidase genes from other speci es. Sequence similarity and divergence percentages between the deduced amino acid sequences of the psr3 clones and other beta-glycosidases s uggests that they should be included along with two other Brassicaceae genes in a distinct subfamily of the EGA glycosidase gene family. The presence of an endoplasmic reticulum retention signal at the carboxy terminus indicates the likely cellular location of PSR3.2. The possibl e metabolic and regulatory roles of this enzyme during the P-i-starvat ion response are discussed.