In a comparative study of housekeeping genes of Pseudomonas aeruginosa and Escherichia coli, the nucleotide sequence of a proline biosynthetic gene, proC, of P. aeruginosa has been determined. The subunit molecular mass (approximately 29 kDa) and the N-terminal amino acid sequence of purified delta 1-pyrroline 5-carboxylate reductase, the proC gene product, were in agreement with the proC nucleotide sequence. A survey of pairs of isofunctional genes from P. aeruginosa and E. coli reveals that within each pair, translated genes (including proC) have diverged more strongly than have untranslated genes specifying ribosomal or transfer RNAs. The translated genes, but not the untranslated ones, have a G + C content that is typical of the respective genomic G + C contents.