Schematic representation of the exchange of the native RIB promoters with a promoter replacement cassette (PRC). (A) depicts the native RIB locus. By homologous recombination the PRC (B) is inserted into the locus thereby deleting 200 bp upstream of the coding sequence. Transient expression of the Cre-recombinase leads to the excision of the Zeocin marker cassette (ZeoR), resulting in (C). In consequence the RIB gene is expressed in its native locus under the control of the constitutive glyceraldehyde-3-phosphate dehydrogenase promoter (PGAP) and no marker remains with the construct. RIBx Up: 5' untranslated region upstream of the corresponding RIB gene.