In Sinorhizobium meliloti, the nodD3 gene is transcriptionally controlled by two promoters, P1 and P2. Under P1, there is a 660 bp sequence including a small open reading frame, ORF2, followed by the nodD3 coding region. Genetic analysis using the different deletions on the 3' ends of P1 downstream sequence showed that the downstream sequence +1-+125nt is essential for P1 expression. Complementation, mutations and nodulation tests demonstrated that the ORF2 auto-represses P1 expression, while the P1 downstream sequence +1-+125nt counteracts it.