Chrysler Corp. will shut down its car assembly plant in St. Louis and lay off 3,700 workers in a move that's likely to worsen already souring relations between the company and its union. Thursday's announcement, the latest in a series of moves by U.S. automakers to cut capacity in the face of slumping sales, makes it likely that job security will be the union's top priority when it opens contract negotiations with the nation's Big Three automakers this summer. Chrysler said its St. Louis No. 1 plant will close Sept. 21, one week after its existing contract with the United Auto Workers expires. As previously announced, Chrysler will eliminate one shift at the plant on Feb. 12. The company said it will transfer production of the Dodge Daytona to its Sterling Heights, Mich., assembly plant, and shift assembly of the Chrysler LeBaron coupe and convertible to its Newark, Del., plant. The St. Louis plant will be the third Chrysler has closed since late 1988. It was announced just one day before the company closes a Detroit plant and puts 1,700 other workers on indefinite layoff. The move angered United Auto Workers officials. ``Workers and their unions don't design the cars or develop the marketing plans,'' UAW President Owen Bieber and Vice President Stan Marshall, head of the union's Chrysler department, said in a statement. ``Senior management calls those shots, but when they miscalculate or screw up, they all stay in place while the workers are told to hit the street,'' they said. Chrysler spokesman Doug Nicoll said the timing of the St. Louis plant closing was dictated by the UAW contract, which expires Sept. 14. That pact doesn't allow the company to close a plant because of product transfers. It says plants may be shut down only if sales of its products slump badly. Chrysler used that as justification for today's closing of its Jefferson Avenue assembly plant in Detroit. That factory makes Dodge Omni and Plymouth Horizon subcompact cars, whose sales have fallen by more than 50 percent from their heyday of about 208,000 in 1986. The UAW has contested that closing, saying Chrysler didn't do enough to keep the design of the small cars fresh and market them properly. Some of the 1,700 workers laid off from Jefferson Avenue will be offered jobs at Jefferson North, a plant under construction next to the existing factory. That plant will begin producing a sport-utility vehicle in late 1992. Chrysler is shutting the St. Louis plant in two stages. Beginning Feb. 12, it will eliminate one shift, putting 1,900 of its 3,700 workers on indefinite layoff. In September, it will lay off the remaining 1,800 workers and close the plant. The announcement couldn't have come at a worst moment, said Harley Shaiken, a labor professor at the University of California at San Diego. ``I think this coming on top of Jefferson is going to create a lot of anxiety and hostility in the industry going into negotiations,'' he said. The UAW opens talks with General Motors Corp., Ford Motor Co. and Chrysler in July. UAW President Owen Bieber said negotiating priorities will be worked out at a special union convention in May in Kansas City, Mo., but job security was more than likely to be the top issue. Michigan State University labor professor Dale Brickner agreed. ``Job security is going to be the barn burner,'' Brickner said. ``If we had strikes this fall, one or more, it certainly would not surprise me.'' Nicoll, the Chrysler spokesman, said the Delaware and Michigan plants would be idled in November to retool for the new cars and would resume production in December. Employment at each plant will increase by a few hundred workers, with laid off St. Louis workers getting the first cracks at the new jobs. The St. Louis closing is one in a series of temporary or indefinite shutdowns or production cuts at Big Three assembly plants nationwide. About 15,400 workers will have lost their jobs at 11 GM or Chrysler assembly plants because of indefinite layoffs between January and September. ``This isn't the last,'' Shaiken said. ``I think we will see something else at GM, particularly as contract time approaches.'' The closures come as auto sales slump and capacity at Japanese-owned plants grows. Within the last year, Honda Motor Co. Ltd. has opened a second assembly plant in Ohio and Nissan Motor Corp. has announced a huge expansion of its factory in Smyrna, Tenn. Since 1986, the best year in automotive sales history, GM and Chrysler have lost market share while Ford and Japanese companies, with the help of their new U.S. plants, have increased their shares.