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Objective The relationships between sequences, structures and functions of the fungal NR-PKS PT domains for C2-C7 cyclization were analyzed with all known fungal NR-PKS sequences.Methods Bioinformatic methods, including phylogenetic analysis, structure modeling and evolutional conservation analysis, were applied to systematicly study the PT domains.Results The C2-C7 catalytic pockets of PT domains were lack of the finger-like regions and the catalytic dyads located at the bottom of the pockets.Comparing PT domains for C2-C7 cyclization in different groups, most of the cavity lining residue (CLR)sites were common, while the regional CLR site mutations resulted in the differentiations of the cavity shapes and volumes corresponding with structures of the representative compounds.Conclusion The conservative residues in PT sequences for C2-C7 cyclization were responsible for the cyclization functions and the evolution of the key residues resulted in the differentiations of cyclization functions attributed to the changes of the cavity sizes.The findings may help to better understand the C2-C7 cyclization mechanisms of PT domains and even predict the structural types of the aromatic polyketide products.