MOLECULAR ANALYSIS OF THE GENE FAMILY OF THE SIGNAL RECOGNITION PARTICLE (SRP) RNA OF TOMATO

The sequence variants of the signal recognition particle (SRP) RNA gen e family from four tomato cultivars have been isolated and characteriz ed which indicated the existance of SRP RNA pseudogenes. Sequence anal ysis revealed two conserved sequence motifs in the upstream region, a TATA-like box and an upstream sequence element (USE), 'TCCCACATCG', bo th located at a conserved distance to the transcription start point. T hese elements are identical to the DNA-dependent RNA polymerase III (p ol III)-specific promoters of U-rich small nuclear RNA (UsnRNA) genes of plants. Moreover, T-rich stretches are found at the 3' end of the c oding regions of the SRP RNA genes which could act as typical pol III termination signals. These findings and recent results from site-direc ted mutation analysis of the SRP RNA genes from Arabidopsis thaliana i ndicate that, in contrast to mammalian systems, plant pol III SRP RNA genes are most probably regulated by external promoter elements. Accor ding to the identical promoter organization between plant U3-, U6snRNA , MRP-like RNA and SRP RNA genes, one can group these genes into the ' pol III(EXT)USE' subclass of externally regulated USE-dependent pol II I genes.