Directors of both banks have approved a definitive agreement for First Interstate Bank of Oregon, a unit of First Interstate Bancorp, Los Angeles, to acquire Jefferson State Bank of Oregon . . . The Wire Service Guild, which represents United Press International employees nationally, has filed an unfair labor practice charge with the National Labor Relations Board accusing UPI of engaging in "surface bargaining" to block a new contract with the guild . . . Shearson Lehman Bros. Holdings and E. F. Hutton Group said five suits have been filed against them by Hutton shareholders for allegedly failing to disclose important information about merger talks in 1986 . . . Standard & Poor's removed Southmark Corp.'s debt issues from its Creditwatch list, with negative implications. The Texas financial services firm had been on the list since Nov. 18, when it began merger negotiations with ICH, a Louisville, Ky., insurance holding company. Merger talks ended Dec. 14 . . . Transamerica Insurance Co., based in Los Angeles, completed purchase of a 51% interest in River Thames Insurance Co., a property casualty reinsurer, from Sedgwick Group PLC, London, for $24.5 million cash.