HthA, a putative DNA-binding protein, and HthB are important for fruiting body morphogenesis in Myxococcus xanthus

In response to starvation, Myxococcus xanthus initiates a developmental programme that results in the formation of spore-filled multicellular fruiting bodies. Fruiting body formation depends on the temporal and spatial coordination of aggregation and sporulation and involves temporally and spatially coordinated changes in gene expression. This paper reports the identification of two genes, hthA and hthB, that are important for fruiting body formation. hthA and hthB are co-transcribed, and transcription of the two genes decreases strongly during development. Loss of HthA and HthB function results in delayed aggregation, a reduction in the level of sporulation, and abnormal developmental gene expression. Extracellular complementation experiments showed that the developmental defects caused by loss of HthA and HthB function are not due to the inability to synthesize an intercellular signal required for fruiting body formation. HthA, independent of HthB, is required for aggregation. HthB, alone or in combination with HthA, is required for sporulation. HthA is predicted to contain a C-terminal helix-turn-helix DNA-binding domain. Intriguingly, the N-terminal part of HthA does not exhibit significant amino acid similarity to proteins in the databases. The HthB protein lacks homologues in the databases. The results suggest that HthA is a novel DNA-binding protein, which regulates transcription of genes important for aggregation, and that HthB, alone or in combination with HthA, stimulates sporulation.