Genetic Variability in the Hop-Tolerance horC Gene of Beer-Spoiling Lactic Acid Bacteria

horC is often targeted in rapid molecular tests for lactic acid bacteria (LAB) beer-spoilage ability. Discovery of a 27 bp horC deletion in a LAB unable to spoil beer led us to examine 27 horC(+) isolates across two LAB genera and seven species to assess horC conservation. Nineteen (70%) of the isolates had the gap, with eight (30%) additionally containing a 3 bp gap yielding a cysteine excision and 14 conserved amino acid substitutions, thus defining three horC orthologs. Pediococcus-specific horC point mutations were also found. horC could not be sequenced in four cases (15%), pointing to horC paralog(s) indistinguishable by the original multiplex polymerase chain reaction (PCR). In contrast, the sequence of the putative horC transcriptional regulator horB was uniformly conserved, suggesting horB is essential for horC function. Modeling of protein structure and hop-compound binding revealed the horC orthologs produce proteins with similar function. Transcriptional analysis, however, found that between and within orthologs, horC expression was not proportional to the hops level present. Transcription analysis also suggested that horB repressed horC in the absence of hops. Thus, it is concluded that different horC orthologs are not uniformly active in response to hops, and targeting short gene sequences during PCR testing for LAB beerspoilage potential is problematic.