Detection of a Hop-Tolerance Gene horA Insertion Variant in Lactic Acid Bacteria That Results in a Truncated HorA Lacking the Walker B Motif Necessary for Transport Function

The hop-tolerance gene horA frequently found in beer-spoilage lactic acid bacteria (LAB) was investigated for sequence variability. Although the horA gene was found to have less sequence variability relative to the LAB hop-tolerance gene horC, a sequence insertion in horA in some isolates resulted in early truncation of HorA translation. This truncated HorA was found in LAB both capable and incapable of growth in beer. Protein modeling revealed that the truncated HorA may retain some capacity to bind and sequester hop iso-alpha-acids but lacks the transport function essential for moving hop compounds out of the cell. Sequence analysis of LAB plasmids that contain horA revealed a high level of conservation in all of the genes comprising the horA gene cassette. Assessing whether a LAB isolated in a brewery setting is capable of making a full-length protein with intact hop transport function or a truncated HorA requires redesign of commonly used horA polymerase chain reaction primers to target detection of horA both with and without the sequence insertion.