The Rubisco small subunit gene as a paradigm for studies on differential gene expression during plant development

The ribulose 1,5-bisphosphate carboxylase small subunit (rbcS) in higher plants is encoded by a small multigene family. Members of the gene family contain 1-3 introns. The rbcS mRNA is differentially distributed in various plant organs. It is most abundant in leaves, less so in stems and other photosynthetic organs, and almost undetectable in roots. In leaves, the rbcS mRNA level is greatly increased by light through transcriptional control of the genes. Ti-mediated gene transfer experiments have demonstrated that the pea rbcS-E9 gene retains light-regulated expression in transformed petunia calluses and in leaves of transgenic petunia and tobacco plants. A 33-base pair sequence around the TATA box region has been shown to be involved in the light-inducibility of the rbcS-E9 gene in transformed calluses. In transgenic petunia plants, the experiments thus far have shown that 352 base pairs of 5' upstream sequence is sufficient for light-inducibility, as well as for leaf-specific expression. Further experiments in progress will help to identify and characterize other cis-acting elements involved in the differential expression of the rbcS genes.

Lysogeny in Rhizobium Leguminosarum and Rh. Trifolii

Seven lysogenic strains-three of Rh. legumino8urum and four of Rh. trifoliiwere identified among a total of 54 strains from these two species and Rh. phaseoli. The Rh.leguminosarum strains may be multiply lysogenic; two of them also produce lethal agents resembling bacteriocins in their effect. Lysogeny was confumed by standard criteria of ultraviolet light inducibility, self.immunity, and reductive ability. Lysogenic conversion involving symbiotic characteristics was not observed. A minimum of six different temperate phages and two clear-plaque-forming variant phages were identified. The host range includes strains of the above three rhizobial species, but not of Rh. melilotii. Variation in host specificity of some phages following host passage is under investigation.