LENGTH OF POLY-CYTIDINE REPEATS CONTROLS THE PHASE-VARIABLE EXPRESSION OF THE FIMBRIAL PROTEIN IN AVIBACTERIUM PARAGALLINARUM

The FlfA fimbrial protein is an important virulence factor and vaccine candidate of Avibacterium paragallinarum. Although all Taiwanese field strains of A. paragallinarumwere shown to contain an intact flfA gene, only some of them express the flfA protein in vitro. The aim of this study was to investigate how flfA expression is controlled in A. paragallinarum. The results of quantitative reverse transcription-polymerase chain reaction, 5′ rapid amplification of cDNA ends, and sequencing analyses showed that flfA expression is controlled by the length of a poly-cytidine (poly-C) tract that is located at the promoter region of flfA. In vitro passage of A. paragallinarum in chickens led to a change in the length of the poly-C tract and subsequently turned off flfA expression. These findings are valuable for understanding the mechanism of pathogenicity of A. paragallinarum.