1.E.4 The LydA Holin (LydA Holin) Family
See 1.E for a generalized description of Holins. Amber and deletion mutants have been used to assign functions in cell lysis to three late genes of bacteriophage P1. Two of these genes, lydA and lydB of the dar operon, are 330 and 444 bp in length, respectively, with the stop codon of lydA overlapping the start codon of lydB (Schmidt et al. 1996). The third, gene 17, is 558 bp in length and is located in an otherwise uncharacterized operon. A search with the predicted amino acid sequence of LydA for secondary motifs revealed a holin protein-like structure. Comparison of the deduced amino acid sequence of gene 17 with database sequences revealed homologies with the proteins of the T4 lysozyme family. To study the effect of gp17, LydA, and LydB in vivo, their genes were cloned in a single operon under the control of the inducible T7 promoter, resulting in plasmid pAW1440 (Schmidt et al. 1996). A second plasmid, pAW1442, is identical to pAW1440 but has lydB deleted. Induction of the T7 promoter resulted in a rapid lysis of cells harboring pAW1442. In contrast, cells harboring pAW1440 revealed only a small decrease in optical density at 600 nm compared with cells harboring vector alone. The rapid lysis phenotype in the absence of active LydB suggests that this novel protein might be an antagonist of the holin LydA.
The lysis genes of Burkholderia phage ST79 encode a holin (TC# 1.E.4.1.4), peptidase M15A or endolysin, lysB and lysC. Each gene and combinations were cloned into Escherichia coli and the lytic effects were measured. Co-expression of holin and peptidase M15A showed the highest lysis activity. Expression of holin, lysB/C or holin, peptidase M15A, lysB and lysC lysed the E. coli membrane whereas peptidase M15A alone did not. The predicted transmembrane structures of holin and lysB/C indicated they could insert into the bacterial membrane to form pores, affecting cell permeability and causing lysis (Khakhum et al. 2016Khakhum et al. 2016).