Molecular characterization of a bacteriophage (Chp2) from Chlamydia psittaci

Comparisons of the proteome of abortifacient Chlamydia psittaci isolates fr om sheep by two-dimensional gel electrophoresis identified a novel abundant protein with a molecular mass of 61.4 kDa and an isoelectric point of 6.41 . C-terminal sequence analysis of this protein yielded a short peptide sequ ence that had an identical match to the viral coat protein (VP1) of the avi an chlamydiaphage Chp1, Electron microscope studies revealed the presence o f a 25-nm-diameter bacteriophage (Chp2) with no apparent spike structures. Thin sections of chlamydia-infected cells showed that Chp2 particles were l ocated to membranous structures surrounding reticulate bodies (RBs), sugges ting that Chp2 is cytopathic for ovine C. psittaci RBs, Chp2 double-strande d circular replicative-form DNA was purified and used as a template for DNA sequence analysis. The Chp2 genome is 4,567 bp and encodes up to eight ope n reading frames (ORFs); it is similar in overall organization to the Chp1 genome. Seven of the ORFs (1 to 5, 7, and 8) have sequence homologies,vith Chp1, However, ORF 6 has a different spatial location and no cognate partne r within the Chp1 genome. Chlamydiaphages have three viral structural prote ins, VP1, VP2, and VP3, encoded by ORFs 1 to 3, respectively. Amino acid re sidues in the phi X174 procapsid known to mediate interactions between the viral coat protein and internal scaffolding proteins are conserved in the C hp2 VP1 and VP3 proteins, We suggest that VP3 performs a scaffolding-like f unction but has evolved into a structural protein.