Myxococcus xanthus viability depends on groEL supplied by either of two genes, but the paralogs have different functions during heat shock, predation, and development

Abstract

Myxococcus xanthus DK1622 contains two paralogous groEL gene loci that possess both different sequences and different organizations within the genome. Deletion of either one of these two genes alone does not affect cell viability. However, deletion of both groEL genes results in cell death unless a complemented groEL1 or groEL2 gene is present. The groEL1 gene was determined to be essential for cell survival under heat shock conditions; a strain with mutant groEL2 caused cells to be more sensitive than the wild-type strain to higher temperatures. Mutants with a single deletion of either groEL1 (MXAN_4895) or groEL2 (MXAN_4467) had a growth curve similar to that of the wild-type strain DK1622 in medium containing hydrolyzed proteins as the substrate. However, when cells were cultured on medium containing either Escherichia coli cells or casein as the substrate, deletion of groEL2, but not groEL1, led to a deficiency in cell predation and macromolecular feeding. Furthermore, groEL1 was found to play an indispensable role in the development and sporulation of cells, but deletion of groEL2 had no visible effects. Our results suggest that, although alternatively required for cell viability, the products of the two groEL genes have divergent functions in the multicellular social life cycle of M. xanthus DK1622.

FIG. 1.

(A) Growth curves of the mutants with single deletions of either groEL1 or groEL2 or a double deletion of groEL1 and groEL2 and complementation with groEL1 (YL0308) or groEL2 (YL0310) and of the wild-type strain DK1622. (B) Quantitative PCR analysis of the expression levels of groEL1 and groEL2 of the wild-type strain DK1622 in CTT medium with an overlaid representative growth curve of the wild-type strain DK1622. The values are shown as levels relative to the expression of groEL1 at the onset of inoculation from a 36-h culture; the error bars represent standard deviations from three independent biological replicates. TM₀, time zero.

FIG. 2.

Growth curves in CT medium (29) for mutants with the groEL1 or groEL2 deletion and for the wild-type strain DK1622.

FIG. 3.

Predation feeding of M. xanthus DK1622, YL0301 (ΔgroEL1), and YL0302 (ΔgroEL2) on an E. coli prey mat. The bar is equal to 5 mm.

FIG. 4.

(A) Development of fruiting bodies of the different groEL mutants and the wild-type strain DK1622 on TPM plates. The bar is equal to 1.5 mm for each panel. (B) β-Galactosidase activity in the wild-type strain DK1622, the fused groEL1-lacZ DK1622 mutant (YL0305), and the fused groEL2-lacZ DK1622 mutant (YL0306) on TPM agar. The datum at each point represents the mean of independent triplicate determinations with standard deviations.