TRANSLATIONAL CONTROL OF PUF OPERON EXPRESSION IN RHODOBACTER-SPHAEROIDES-2.4.1

The puf operon of Rhodobacter sphaeroides 2.4.1 encodes the beta- and alpha-polypeptides of the B875 complex, the L and M polypeptides of th e reaction centre and the pufX gene product. A previous report from th e authors' laboratory indicated the potential existence of a 20-codon open reading frame (orfK, now designated pufK) located immediately ups tream of the pufB structural gene. It is now demonstrated that pufK is translated in vivo and that the specific levels or nature of the rare codons within pufK affect the expression of pufK. Using a series of p ufK-specific mutations, both in trans as lacZ translational fusions an d incorporated into the genome in single copy, evidence has been obtai ned that translation initiation through pufK may be essential to trans lation of pufB, Further, the abundance, quality and distribution of ra re codons within pufK may serve to 'gate' the entry of ribosomes at pu fB. The data also suggest that translation of pufB is uncoupled from t hat of pufA, with the latter capable of being produced in excess of th e former. It is also revealed that the secondary structure at the 5' e nd of the large and small puf transcripts may play a role in mRNA stab ility and that stability of the small puf transcript is independent of translation.