Protelomerases are Functional Analogues of Lambdoid Phage Integrases and Belong to the Family of Site Specific Tyrosine Recombinases

At the lysogenic stage, temperate bacteriophages are generally integrated into the bacterial chromosome. The integration is promoted by phage-encoded enzymes called integrases, which mediate unidirectional site-specific recombination between the DNA recognition sequences of the phage and its host. Integrases may be grouped into two major families, the tyrosine recombi-nases and the serine recombinases, based on their mode of catalysis (Groth and Calos 2004). Members of the tyrosine family are, e.g., the integrases of the lambdoid phages P22 and HK022. Contrary to most temperate phages, the prophages of N15, PY54 and ^KO2 are not integrated into the bacterial chromosome but replicate as linear plasmids with covalently closed ends

Telomerase

Fig. 3 Alignment of the TelN (N15), TelY (PY54) and TelK (^KO2) protelomerases. The positions of the boxes A, B and C are indicated. Arrowheads show the pentad "RKhRH" plus the catalytically active tyrosine residue, which are conserved in many tyrosine re-combinases. The middle histidine in RKhRH is less common than the other residues of this motif, and is a lysine in the PY54 protelomerase and a methionine in the N15 and 4>KO2 protelomerases. A possible binding motif for divalent cations is also shown

Fig. 3 Alignment of the TelN (N15), TelY (PY54) and TelK (^KO2) protelomerases. The positions of the boxes A, B and C are indicated. Arrowheads show the pentad "RKhRH" plus the catalytically active tyrosine residue, which are conserved in many tyrosine re-combinases. The middle histidine in RKhRH is less common than the other residues of this motif, and is a lysine in the PY54 protelomerase and a methionine in the N15 and 4>KO2 protelomerases. A possible binding motif for divalent cations is also shown

(telomeres). However, comparable to phage integrases of other temperate phages, N15, PY54 and ^KO2 possess an enzyme (protelomerase) essential for establishing the lysogenic life cycle. This occurs by conversion of the linear phage genome into the linear plasmid in which the protelomerase is responsible for the generation of the plasmid hairpin ends.

The phage-encoded protelomerases of more than 70 kDa are much larger than most phage integrases, which have sizes between 30 and 40 kDa. Alignments revealed 77% sequence identity between the N15 (TelN) and ^KO2 (TelK) proteins, while the PY54 protelomerase TelY is approximately 41 % identical to TelN and TelK (Fig. 3). The overall homologies to phage integrases as the \ integrase are low. Nevertheless, three major clusters, designated boxes A, B and C, and a pentad ("RKhRH") conserved in many integrases are also present in the protelomerases (Esposito and Scocca 1997; Groth and Calos 2004). Furthermore, the relationship between protelomerases and integrases is corroborated by the facts that in both enzymes a tyrosine residue is pivotal for the catalytic activity, and that during catalysis the protelomerases are transiently linked to a 3'-phosphoryl group of the target DNA (see Sect. 3.2.5).

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