CHARACTERIZATION OF A SYNECHOCOCCUS SP STRAIN PCC-7002 MUTANT LACKINGPHOTOSYSTEM .1. PROTEIN ASSEMBLY AND ENERGY-DISTRIBUTION IN THE ABSENCE OF THE PHOTOSYSTEM-I REACTION-CENTER CORE COMPLEX

A Synechococcus sp, strain PCC 7002 Delta psaAB::cat mutant has been c onstructed by deletional interposon mutagenesis of the psaA and psaB g enes through selection and segregation under low-light conditions. Thi s strain can grow photoheterotrophically with glycerol as carbon sourc e with a doubling time of 25 h at low light intensity (10 mu E m(-2) s (-1)). No Photosystem I (PS I)-associated chlorophyll fluorescence emi ssion peak was detected in the Delta psaAB::cat mutant. The chlorophyl l content of the Delta psaAB::cat mutant was approximately 20% that of the wild-type strain on a per cell basis. In the absence of the PsaA and PsaB proteins, several other PS I proteins do not accumulate to no rmal levels. Assembly of the peripheral PS I proteins PsaC, PsaD, PsaE , and PsaL is dependent on the presence of the PsaA and PsaB heterodim er core. The precursor form of PsaF may be inserted into the thylakoid membrane but is not processed to its mature form in the absence of Ps aA and PsaB. The absence of PS I reaction centers has no apparent effe ct on Photosystem II (PS II) assembly and activity. Although the mutan t exhibited somewhat greater fluorescence emission from phycocyanin, m ost of the light energy absorbed by phycobilisomes was efficiently tra nsferred to the PS II reaction centers in the absence of the PS I. No light state transition could be detected in the Delta psaAB::cat strai n; in the absence of PS I, cells remain in state 1. Development of thi s relatively light-tolerant strain lacking PS I provides an important new tool for the genetic manipulation of PS I and further demonstrates the utility of Synechococcus sp. PCC 7002 for structural and function al analyses of the PS I reaction center.