Page i was not in the other copies of this thesis. -Digitization Centre / Tn5 mutagenesis was used to generate four independent mutants of Rhizobium meliloti that were unable to grow on protocatechuate (Pca-). Two of the Pca- mutations were mapped to a region of the second symbiotic megaplasmid (pRmeSU47b) previously shown to be required for growth on protocatechuate. This pca locus was shown to consist of the first five structural genes of the protocatechuate branch of the B-ketoadipate pathway, in the order pcaDCHGB. This gene order is the same as determined for Agrobacterium tumefaciens. An additional reading frame with homology to LysR-type regulators was found to be upstream of, and transcribed divergently to the pcaDCGHB operon. This is likely to fulfil the same role as the regulatory gene, pcaQ, of A. tumefaciens. A cosmid plasmid which carried these pea genes failed to complement the Pca- phenotype of a strain carrying a 300 kb megaplasmid deletion encompassing this pea locus. This implies that another pca locus, perhaps pcaIJ, is present within the deleted region of the rnegaplasmid. Two Pca- Tn5 insertions which did not map to the megaplasmid locus were isolated. One of these insertions appears to be in a catalase gene. / Thesis / Master of Science (MS)