This is one of a series of pBluescript­based YC­type (centromeric) yeast shuttle vectors (ATCC 77142­77145) differing in the yeast selectable marker gene. It encodes the beta galactosidase alpha peptide for blue­ white color detection of inserts and also has the T7 and T3 promoters for in vitro RNA synthesis and priming sites for sequencing. It is useful in plasmid shuffling experiments. It was constructed by inserting a 1.112 kb fragment containing the URA3 gene into the NdeI site of the pRSS56 vector and also inserting a cassette containing CEN6 and the ARS associated with histone 4 (ARSH4) into the AatII site of the same vector. All ends were blunted. The pRSS56 vector was constructed by ligating a PvuI fragment (bp 498­2412) of pBluescript KS+ and the fragment from the unique NdeI and AatII sites between bla and f1 origin of pBS(+). The order of the major features in this plasmid is: URA3 ? f1 ori (NaeI) ? T7 promoter ? lacZ?/MCS ? T3 promoter ?pMB1 ori ? bla ? CEN6 ? ARSH4.