PORTLAND – Unitil officially closed its $160 million purchase of Maine-based Northern Utilities and Massachusetts-based Granite State Gas Transmission.

Company officials marked completion of the deal with a ceremony in Portland at the offices of Northern Utilities, which serves 52,000 customers in Maine and New Hampshire.

Unitil bought Granite State Gas Transmission and Northern Utilities from NiSource Inc., based in Merrillville, Ind. Northeast Utilities is one of the primary customers of Granite State, which operates 86 miles of underground natural gas pipeline in Massachusetts, Maine and New Hampshire.

Unitil, which is based in Hampton, N.H., is a utility holding company which had about 115,000 customers in New Hampshire and Massachusetts before the deal was completed.