1.M.2. The Spanin2 (Spanin2) Family
Under usual laboratory conditions, lysis by bacteriophage lambda requires only the holin and endolysin genes, but not the Rz and Rz1 genes, of the lysis cassette. Defects in Rz or Rz1 block lysis only in the presence of high concentrations of divalent cations. While Rz and Rz1 equivalents have been identified in, for example, T7 and P2 phage, many phages, including such well-studied classic phages as T4, P1, T1, Mu and SP6, have been reported to lack annotated Rz/Rz1 equivalents.Â Possibly these proteins are not required for cell lysis in some instances, or alternatively, other non homologous proteins may provide this disruptive function. The prototype for the Spanin2 Family is the U-spanin of phage T1.
Summer et al. (2007) reported that a search strategy based primarily on gene arrangement and membrane localization signals, rather than sequence similarity, revealed that Rz/Rz1 equivalents may be nearly ubiquitous among phages of Gram-negative bacteria.Â In fact, 120 of 137 phages possessed genes that fit the search criteria. In the case of T4, a deletion of a non-overlapping gene pair, pseT.2 and pseT.3, considered as Rz/Rz1 equivalents, resulted in the same divalent cation-dependent lysis phenotype that was known to characterized mutants of T7 that lacked the Rz/Rz1 system. Remarkably, in T1 and six other phages, Rz/Rz1 pairs were not found, but a single gene encoding an outer membrane lipoprotein with a C-terminal transmembrane domain capable of integration into the inner membrane was identified. These proteins were named spanins, since they are predicted to span the periplasm, providing a physical connection between the inner and outer membranes. The T1 spanin gene was shown to complement the lambda Rz-Rz1- lysis defect, indicating that spanins function as Rz/Rz1 equivalents. The widespread presence of Rz/Rz1 or their spanin equivalents in phages of Gram-negative hosts suggests a strong selective advantage and that their role in the ecology of these phages is greater than that inferred from the mild laboratory phenotype (Summer et al. 2007).