Figure 3.

(A) Physical map and intron–exon structure of the Volvox nitrate reductase gene (18) (GenBank accession no. X64136). Plasmid 4 carries a 5′-deleted part of the original nitrate reductase gene with five base pair exchanges with respect to the parent gene, one of them repairing the ruined intron–exon splice site (which is marked by an asterisk). The primers used in genomic PCR are indicated (P1 and P2). P1 matches only the genomic sequence and not the plasmid sequence. For clarity, only the insert of the circular plasmid 4 is shown. (B) Part of the genomic sequence of the nitrate reductase gene in the original Volvox strain 153–48 (parent strain), part of the sequence of the 5′-deleted nitrate reductase plasmid 4 (plasmid), and part of the genomic sequence in the recombinant Volvox strains HRN-1 and HRN-2 (transformants), showing a homologous recombination event. E, EcoRI; H, HindIII; S, SalI.