YCtype centromere vector permitting visual detection of recombinants and production of ssDNA in E. coli. Contains promoters for in vitro RNA synthesis, priming sites useful for sequencing, and encodes the lacZ alpha (lacZ) peptide. pRSS56, constructed by ligating a PvuI fragment (bp 4982412) of pBluescript KS+ to a PvuI fragment (bp 2850 730) of pBS(+), contains the KS MCS from pBluescript KS(+) and the unique NdeI and AatII sites between bla and f1 origin of pBS(+). A genomic HindII/PstI fragment (1.002 kb) containing the TRP1 gene was inserted into the NdeI site and a cassette containing CEN6 and the ARS associated with histone 4 (ARSH4) was inserted into the AatII site of pRSS56. All ends were blunted. An EcoRI site in the TRP1containing fragment (external to the coding sequence) was destroyed. The order of the major features in this plasmid is: TRP1f1 ori (NaeI) T7 promoter lacZ/MCS T3 promoter pMB1 ori bla CEN6 ARSH4.