General Motors Corp. (GM) plans to invest $500 million to upgrade five Michigan assembly and parts plants, adding 200 to 300 jobs, according to published reports.

The automaker, joined by state officials, was scheduled to announce the plans Tuesday afternoon at a plant in Pontiac, one of the facilities to see part of the investment, The Detroit News and the Detroit Free Press reported.

GM was expected to announce that it will upgrade the Pontiac Assembly Center and Pontiac Stamping Plant, as well as an engine plant in Romulus, the Ypsilanti transmission plant and the Grand River Assembly plant in Lansing, the newspapers reported, quoting sources familiar with the announcement.

GM spokesman Dan Flores declined to give any details other than to say the announcement will have a "positive impact" on the five sites and that four of them are in the greater Detroit area.

Joe Spielman, GM's general manager of North American manufacturing, was to be joined at Tuesday's news conference at GM's Pontiac Centerpoint Campus by Gov. Jennifer Granholm, Oakland County Executive L. Brooks Patterson, Wayne County Executive Robert Ficano and Pontiac Mayor Clarence Phillips.

The newspapers said the Pontiac assembly plant would be upgraded to build GM's new full-size pickup trucks, which are to go on sale in the fall. The Grand River plant in Lansing also will see new investment. It builds the Cadillac CTS and STS sedans and the SRK sport utility vehicle.

The investment would be welcome news in a state that relies heavily on the auto industry, because it is a signal that GM intends to keep the plants open.

GM announced plans in November to cut 30,000 hourly jobs and close nine plants and three parts facilities in North America by 2008. Meanwhile, Ford Motor Co. has said it plans to cut up to 30,000 jobs and close 14 plants in North America by 2012, and Delphi Corp., GM's former parts division, filed for bankruptcy in October and is seeking steep wage cuts from workers.