Rates of purine biosynthesis are far greater in nodules than in other tissues of cowpea. Consequently, there is greatly increased expression of purine biosynthesis (pur) genes in nodules. In this project the transcriptional regulation of pur2 and pur5 genes in cowpea (Vigna unguiculata (L.) Wapl.) root nodules were investigated. These genes encode GAR synthetase and AIR synthetase respectively which catalyse the 2^nd and 5^th steps of the purine biosynthesis pathway. Two approaches were taken to study transcriptional regulation of these pur genes. Firstly, the effects of the fixed nitrogen and products of nitrogen fixation on transcription of pur2 and pur5 were investigated. Secondly, the promoter of the pur2 gene was studied in tobacco (Nicotiana tobacum) to identify the specific regions important for induction of constitutive gene expression using promoter deletion studies. While tobacco is not a leguminous plant it is likely that the regulation of pur genes in vegetative tissues is the same for cowpea. In the first part of this study, exogenous application of alanine or ammonium sulphate caused no effect on pur5 expression in mature (nitrogen fixing) nodules. Similarly, the effect of N₂ deficiency on pur5 expression in nodules of cowpea plants grown in Argon: Oxygen atmosphere (nitrogen fixation absent) was the same for those arown. in air (where nitrogen fixation is present). This suggests that the products of nitrogen fixation are not involved in regulating pur5 expression, and that regulation is controlled by other developmental and possibly hormonal factors. In contrast, expression of pur2 in nodules of plants grown in Argon: Oxygen was delayed compared to those grown in air. This suggests that there are two signals or control mechanisms involved in regulation of pur2 (the onset of nitrogen fixation and other developmental signals) and that these may interact to induce expression of the gene. In the second part of this study, the promoter of the pur2 gene was further characterised in tobacco to determine regions important for constitutive expression. Promoter deletion studies suggested that the expression of pur2 in tobacco leaves appears to be under positive control. Two regions important for constitutive expression of pur2 were identified in the promoter as between -834 and -992 bp and between - 1184 and -1899 bp upstream from the start of transcription. It is likely that these regions of the promoter contain cis-acting elements to which transcription factors bind and interact with RNA polymerase to initiate transcription.