The gene for Saccharomyces cerevisiae inorganic pyrophosphatase, PPA, has been cloned by hybridization of "long" oligonucleotide probes with both cDNA and genomic S. cerevisiae libraries. The nucleotide sequence of 1612 bp from a genomic subclone that includes the entire coding region gives a deduced amino acid sequence that has nine differences (out of a total of 286 residues) from the previously published amino acid sequence that was determined directly. The codon usage in PPA is as expected for a "highly expressed" yeast gene. The upstream region contains a poly dA/dT sequence that might comprise a constitutive promoter. The PPA gene appears to be present in a single copy within the S. cerevisiae genome and has been localized to chromosome II.

The mismatch analysis of PCR-amplified DNA has generally assumed the absence of artificially introduced base substitutions in a significant proportion of the amplification product. This technique, however, differs from the direct sequencing of amplified DNA in that non-specific substitutions will render a molecule useless in analysis. The expected signal-to-noise ratio is heavily influenced by several parameters viz. initial template copy number, number of replication cycles, eventual product yield and the type of experimental system adopted. Mathematical modelling can be used to optimize fragment length with respect to the method applied and suggests as yet undescribed improvements such as partial modification or cleavage to optimize signal detection.

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A cystic fibrosis patient homozygous for the nonsense mutation R553X was identified by mutation screening and the genotype confirmed by DNA sequencing. This patient, the only one described to date who is homozygous for this stop codon in exon 11 of the CFTR gene, is moderately severely affected. Clinical and molecular findings are presented.