FIRST OFF . . .

First-run programming for cable TV took another step forward Wednesday as the USA Network said it plans to launch a weekly series of original TV movies in April, 1989. The first project is “The Zodiac Murders,” based on an unsolved murder case in Northern California. Officials of the USA Network--a basic cable service that reaches more than 45 million homes nationwide and is owned jointly by MCA and Paramount--said the films will be similar to the sort that run on ABC, CBS and NBC. After being shown on cable, the movies will be released on home video.