Isolation of SIP, a protein that interacts with SPB, a possible transcriptional regulatory factor in Rhodobacter sphaeroides.

SPB is a transcriptional factor in Rhodobacter sphaeroides that represses expression of the puf operon under aerobic or semi-aerobic light conditions. Here, we identified a 17,500 Da protein designated SIP (SPB interaction protein) that interacts with SPB, as determined by binding to an SPB-His(x6) fusion protein-Ni column. The SPB-SIP interaction in vivo was confirmed by an immunoprecipitation assay. The level of transcripts and protein of SIP did not differ for all growth conditions tested, indicating that regulation of the SIP-SPB interaction, if any, is not through modulation of sip or spb expression but rather by modification of the proteins.