The hit locus gene product Bd0108 interacts in the periplasm with Bd0109 regulating pilus extrusion and predatory to prey-independent growth-switching in Bdellovibrio bacteriovorus. TGAC_PILOT_LizSockett_Bdellovibrio

Bdellovibrio bacteriovorus are facultatively predatory bacteria, switching between intracellular growth after invasion of other Gram-negative bacteria, and slow prey-independent growth on organic nutrients. The nature of the growth switch mechanism has been elusive, but several researchers have reported that mutations, including a 42bp deletion, in a gene bd0108 unique to Bdellovibrio, were associated with the transition to prey-independent growth in some populations of B. bacteriovorus. Genome sequencing around this locus predicted that it was part of a cluster of Type IVb pilus-associated genes. Type IVa pilus expression is known to be required for prey entry by predatory Bdellovibrio. In this study we have deleted the whole bd0108 gene and studied its phenotype in comparison to a strain with the common 42bp bd0108 deletion. We find that deletion of the whole bd0108 gene abolished pilus production whereas the 42bp deletion caused greater pilus extrusion than wild type. Attempts to similarly delete gene bd0109, which like bd0108 encodes a periplasmic/secreted protein, were not successful, suggesting that it is likely to be essential for Bdellovibrio viability in any growth mode. Bd0109 has a sugar binding YD- repeat motif and an N terminus with a pilus-like motif and was found to interact with Bd0108. In the absence of Bd0108 no pili are produced, but in the presence the 42bp deleted form product of Bd0108, normal regulation of pilus production is altered and abnormal extruded pili are present, composed of the normal Type IVa pilus fibre protein PilA. We propose that Bd0109/Bd0108 interaction regulates pilus production in Bdellovibrio by binding at the cell wall and that the presence (and possibly retraction state) of the Bdellovibrio Type IVa pilus feeds back to alter the growth state of the Bdellovibrio cell. We note that a small RNA is encoded at the hit locus and its transcription is altered in different types of mutation in bd0108.