Experimental screen for the presence of an RNA-coding gene in the guaB-trxA intergenic region. (a) Sequence alignment of the guaB-trxA (guaB: sequence not shown, located upstream of yfr1) intergenic region visualises the conserved yfr1 gene labeled by the bar above the alignment and its transcriptional initiation site in three of the analyzed strains (MED, MED4; MIT, Prochlorococcus strain MIT 9313; WH8, Synechococcus sp. WH 8102) but not in Prochlorococcus strain SS120 (SS1). Transcriptional initiation sites (TIS) and the deduced -10 elements are indicated. (b) Northern blots show a signal for Yfr1 at a size of 54, 56 and 57 nucleotides (nt) for MED4, WH 8102 and MIT 9313, respectively. No signal with RNA from SS120 confirms the absence of this gene in this strain, as was predicted from the sequence data. (c) Predicted secondary structures of Yfr1 in MED4, MIT 9313 and WH 8102 by MFOLD .