Enterobacteria satellite phage P4 gene alpha (P10277, 777 aa, ~84.9 kDa) encodes the multifunctional replication initiator, DNA helicase, and DNA primase protein gpAlpha [PMID:3309336, PMID:2183201, PMID:8253092]. Unlike morphogenesis and lysis functions (which P4 borrows from its helper phage P2), P4 DNA replication is autonomous and relies directly on gpAlpha [PMID:3309336, PMID:8253092].
Ziegelin et al. (1993) demonstrated that the 84.9 kDa gpAlpha polypeptide combines three essential replication activities in a single protein PMID:8253092:
1. N-terminal Prokaryotic DNA Primase Domain (aa 1–426):
- Contains a Cys4 zinc-finger motif (GO:0008270, zinc ion binding) and the conserved -E-G-Y-A-T-A- primase catalytic motif shared with IncI/IncP plasmid primases PMID:1618804.
- Synthesizes short RNA primers (GO:0003896, DNA primase activity; GO:0006269, DNA replication, synthesis of primer) on single-stranded DNA templates initiating at 5'-d(CTG)-3' sites [PMID:1618804, PMID:8253092].
2. Central NTP-Driven DNA Helicase Domain:
- Exhibits ATP/GTP-hydrolyzing (GO:0005524, ATP binding; GO:0016887, ATP hydrolysis activity) 3'→5' DNA helicase activity (GO:0003678, DNA helicase activity) required to unwind the P4 replication origin PMID:8253092.
3. C-terminal Winged-Helix Origin-Binding Domain (P4-OBD, aa 652–777):
- Specifically recognizes Type I iteron direct repeats within the P4 replication origin (ori) and the crr (cis replication region) (GO:0003677, DNA binding; GO:0006260, DNA replication), and is regulated via heterodimerization with the P4 copy-number control protein Cnr (P68666) [PMID:8253092, PMID:11929537].
GO:0003677 (DNA binding), GO:0003678 (DNA helicase activity), GO:0005524 (ATP binding), GO:0006260 (DNA replication), GO:0006269 (DNA replication, synthesis of primer), GO:0008270 (zinc ion binding), and GO:0016887 (ATP hydrolysis activity).GO:0003899 (DNA-directed RNA polymerase activity, IDA PMID:1618804), GO:0016740 (transferase activity), and GO:0016779 (nucleotidyltransferase activity) -> refine to GO:0003896 (DNA primase activity).GO:0004386 (helicase activity) -> refine to GO:0003678 (DNA helicase activity).GO:0000166 (nucleotide binding) -> refine to GO:0005524 (ATP binding).GO:0016787 (hydrolase activity) -> refine to GO:0016887 (ATP hydrolysis activity) and GO:0003678 (DNA helicase activity).GO:0046872 (metal ion binding) -> refine to GO:0008270 (zinc ion binding).GO:0000428 (DNA-directed RNA polymerase complex), which is a false keyword mapping from KW-0240; gpAlpha is a replication primase-helicase, not a subunit of a transcription RNA polymerase holoenzyme.GO:0003896 (DNA primase activity, IDA, PMID:8253092).